#297 - Fix transparent resource dropdown.

The value of the CSS attribute "overflow" [was set to "visible"](https://github.com/gocd/gocd/blob/master/server/webapp/sass/gadgets/dropdowns.scss#L28) (the
correct value) for the enhanced_dropdown class. Then, in the same file
(dropdowns.scss), the value was changed to "hidden", which is the wrong
value.

Chrome behaved differently from Firefox in this case (unknown reason).