fix(common/Gruntfile.js): empty main.css when using compass

Building with `grunt build` generates an empty
`dist/styles/*.main.css` file because grunt is messing up
the grunt target name, i.e. instead of `concat:dist` grunt
is trying to execute `concat:dist/styles/main.css`, and as
there is no target configuration with this name the main.css
gets overwritten with an empty file.

Using `concat:dist` and `cssmin:dist` instead of `concat`
and `cssmin` seems to workaround the problem.