Merge #7920: Switch Travis to Trusty
a33b7c9 travis: temporarily disable qt to avoid timeouts (Cory Fields)
174023c travis: Don't disable writing ccache for pull-requests (Cory Fields)
cf77fcd travis: drop MAKEJOBS=2 for windows compilers (Cory Fields)
9267a47 depends: enable pre-compiled headers for qt (Cory Fields)
06fdffd travis: switch to Trusty (Cory Fields)
a6666b2 depends: mac deploy Py3 compatibility (Wladimir J. van der Laan)