chore(deps): upgrade date-math (#1354)

The diff is quite nice.

```diff
-    "minified": 150810,
+    "minified": 148190,
-    "gzipped": 45487
+    "gzipped": 44792
```

Author: TrySound

.size-snapshot.json

@@ -1,25 +1,25 @@
 {
   "./dist/react-big-calendar.js": {
-    "bundled": 554087,
-    "minified": 170209,
-    "gzipped": 49950
+    "bundled": 548465,
+    "minified": 167635,
+    "gzipped": 49330
   },
   "./dist/react-big-calendar.min.js": {
-    "bundled": 490328,
-    "minified": 150824,
-    "gzipped": 45415
+    "bundled": 484706,
+    "minified": 148190,
+    "gzipped": 44792
   },
   "dist/react-big-calendar.esm.js": {
-    "bundled": 169648,
-    "minified": 81524,
-    "gzipped": 20082,
+    "bundled": 167242,
+    "minified": 79833,
+    "gzipped": 19791,
     "treeshaked": {
       "rollup": {
-        "code": 59035,
-        "import_statements": 1402
+        "code": 57605,
+        "import_statements": 1572
       },
       "webpack": {
-        "code": 62470
+        "code": 62052
       }
     }
   }

package.json

@@ -123,7 +123,7 @@
     "@babel/runtime": "^7.1.5",
     "bootstrap-sass": "^3.4.1",
     "classnames": "^2.2.6",
-    "date-arithmetic": "^3.0.0",
+    "date-arithmetic": "^4.0.0",
     "dom-helpers": "^3.4.0",
     "invariant": "^2.2.4",
     "lodash": "^4.17.11",

src/Agenda.js

@@ -4,7 +4,7 @@ import classes from 'dom-helpers/class'
 import getWidth from 'dom-helpers/query/width'
 import scrollbarSize from 'dom-helpers/util/scrollbarSize'
 
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 import { navigate } from './utils/constants'
 import { inRange } from './utils/eventLevels'
 import { isSelected } from './utils/selection'

src/BackgroundCells.js

@@ -3,7 +3,7 @@ import React from 'react'
 import { findDOMNode } from 'react-dom'
 import cn from 'classnames'
 
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 import { notify } from './utils/helpers'
 import { dateCellSelection, getSlotAtX, pointInBox } from './utils/selection'
 import Selection, { getBoundsForNode, isEvent } from './Selection'

src/DateContentRow.js

@@ -5,7 +5,7 @@ import PropTypes from 'prop-types'
 import React from 'react'
 import { findDOMNode } from 'react-dom'
 
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 import BackgroundCells from './BackgroundCells'
 import EventRow from './EventRow'
 import EventEndingRow from './EventEndingRow'

src/Day.js

@@ -1,7 +1,7 @@
 import PropTypes from 'prop-types'
 import React from 'react'
 
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 import { navigate } from './utils/constants'
 import TimeGrid from './TimeGrid'
 

src/DayColumn.js

@@ -4,7 +4,7 @@ import { findDOMNode } from 'react-dom'
 import cn from 'classnames'
 
 import Selection, { getBoundsForNode, isEvent } from './Selection'
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 import * as TimeSlotUtils from './utils/TimeSlots'
 import { isSelected } from './utils/selection'
 

src/EventCell.js

@@ -1,7 +1,7 @@
 import PropTypes from 'prop-types'
 import React from 'react'
 import cn from 'classnames'
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 
 class EventCell extends React.Component {
   render() {

src/Month.js

@@ -3,7 +3,7 @@ import React from 'react'
 import { findDOMNode } from 'react-dom'
 import cn from 'classnames'
 
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 import chunk from 'lodash/chunk'
 
 import { navigate, views } from './utils/constants'

src/Popup.js

@@ -3,7 +3,7 @@ import React from 'react'
 import getOffset from 'dom-helpers/query/offset'
 import getScrollTop from 'dom-helpers/query/scrollTop'
 import getScrollLeft from 'dom-helpers/query/scrollLeft'
-import dates from './utils/dates'
+import * as dates from './utils/dates'
 
 import EventCell from './EventCell'
 import { isSelected } from './utils/selection'