Add Travis CI support for Linux

[uilianries]

WIP

To avoid many pages as needed because of Travis limitation due 50k log lines, my idea is:

• Run Conan export to ALL recipes.
• Build only the latest layer, but using build policy as "missing", then all requires from other layers will be built before.
• Share the same Conan data cache between all builds, since we can't share those package after closing a Docker container.
• For now we are using "testing" channel only, but we need to think how to promote it, we may use CPT internal helpers.

[uilianries]

Macos support is not working well:

https://travis-ci.com/bincrafters/conan-x11/jobs/226035783#L3829

I gonna remove OSX from travis for now, we can add it again in future.

[SSE4]

we don't need OSX now. great job @uilianries !