Create different variants of the "build" task for gcc and clang

The clang variant remains the same as it was before (i.e. complete
compilation), and the gcc task is reduced to only compiling `pcl_common`
target.
Remove cmake from the install list

This package is guaranteed to be present on Travis
Merge pull request #707 from taketwo/simplify-gcc-task-on-travis

Simplify gcc task on Travis