Copy generated tutorials to Git repository folder (instead of moving)

With `mv` the following error occurs:

mv: cannot move `/home/travis/build/PointCloudLibrary/pcl/build/doc/tutorials/html' to `/home/travis/build/PointCloudLibrary/pcl/build/doc/doxygen/html/tutorials/html': Directory not empty
Merge pull request #779 from taketwo/travis-cp-instead-of-mv

Copy generated tutorials to Git repository folder (instead of moving)