Default USE_PCH to ON and always use that for CI

[t-b]

I really like this PR and I am actually in favor of setting USE_PCH to ON as default value in the CMakeLists.txt files and to enable it for all the travis builds too.

Originally posted by @bourtemb in #526 (comment)

[t-b]

I think we should keep at least one travis job (only compilation without tests) to use USE_PCH=OFF just that we now that it still works.