travis-ci cleanup (#3649)
travis-ci cleanup (#3649)

* travis-ci add more caching

* travis-ci move doxygen to semaphoreci

* travis-ci move linux debug to semaphoreci

kd0aij/qgroundcontrol,CornerOfSkyline/qgroundcontrol,CornerOfSkyline/qgroundcontrol,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,RedoXyde/PX4_qGCS,kd0aij/qgroundcontrol,CornerOfSkyline/qgroundcontrol,RedoXyde/PX4_qGCS,Hunter522/qgroundcontrol,RedoXyde/PX4_qGCS,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,kd0aij/qgroundcontrol,ethz-asl/qgc_asl,ethz-asl/qgc_asl,kd0aij/qgroundcontrol,ethz-asl/qgc_asl,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,CornerOfSkyline/qgroundcontrol,kd0aij/qgroundcontrol,CornerOfSkyline/qgroundcontrol,RedoXyde/PX4_qGCS,RedoXyde/PX4_qGCS,RedoXyde/PX4_qGCS,Hunter522/qgroundcontrol,kd0aij/qgroundcontrol,CornerOfSkyline/qgroundcontrol