Add between operator

* We need this to use the operator in a having clause.
* On Laravel 5.5, this operator was removed by laravel#22182
* On Laravel 5.7 there is a new havingBetween method that we can use instead laravel#26758

Author: Ivan Zazueta

src/Illuminate/Database/Query/Builder.php

@@ -178,7 +178,7 @@ class Builder
      */
     public $operators = [
         '=', '<', '>', '<=', '>=', '<>', '!=', '<=>',
-        'like', 'like binary', 'not like', 'ilike',
+        'like', 'like binary', 'not like', 'ilike', 'between',
         '&', '|', '^', '<<', '>>',
         'rlike', 'regexp', 'not regexp',
         '~', '~*', '!~', '!~*', 'similar to',