Add a test to check for semicolon after import

There should be a semicolon after the `@import` block in the stylesheet. It will work without one, but will prevent additional styles from working.

Props to [Luis Alejandre](http://wpthemedetector.com) for finding.
Add semicolon to the `@import` block

See last commit for details.

Props to [Luis Alejandre](http://wpthemedetector.com) for finding and suggesting a fix.