Use explicit [0-9] instead of \d in go verion regex to make the Ubuntu version of grep happy

Author: mdlavin

.travis.yml

@@ -7,7 +7,7 @@ go:
 before_install:
 - go get -u github.com/kardianos/govendor
 - go get github.com/mitchellh/gox
-- export GO_VERSION=$(go version | cut -d' ' -f3 | grep -Eo '\d+\.\d+\.\d+')
+- export GO_VERSION=$(go version | cut -d' ' -f3 | grep -Eo '[0-9]+\.[0-9]+\.[0-9]+')
 install:
 - govendor sync
 script: