Fix Syntastic deprecation warning

`hasErrorsOrWarningsToDisplay` has been deprecated in Syntastic.
See: https://github.com/scrooloose/syntastic/commit/d629be9

This fixes the warning that appears when opening a new file:
"syntastic: warning: function hasErrorsOrWarningsToDisplay() is
deprecated, please use !isEmpty() instead"
Merge pull request #759 from fitztrev/syntastic-errors

Fix Syntastic deprecation warning