Update ply_io.cpp

Reading a binary PLY file with faces and "vertex_indices" property crashes, if only a PointCloud is provided. Since the polygons_ member is set to 0 and untouched otherwise.

Worked in PCL 1.6, doesn't work right now with 1.7.1.

The ply2pcd sample program provided with PCL crashes as well.
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 #536 from fran6co/fix-ply

Update ply_io.cpp
Merge pull request #707 from taketwo/simplify-gcc-task-on-travis

Simplify gcc task on Travis