Update travis configuration

[marian42]

• Split cpp build and doxygen into separate travis jobs
• Ros project is only built for pullrequests, documentation is only built for pushes to master
• The check for doxygen was moved from the bash script to the travis configuration so that no VM is created if doxygen will not be run
• Remove some unused dependencies (Rviz and doxygen are no longer installed for cpp builds)
• Unfortunately, gazebo is still installed which takes about 90 seconds. This is due to the fact that the --skip-keys option for rosdep is broken.