Fix language and tests in Travis

[dfdx]

1. Travis now supports language: julia entry, so it's possible to replace previous language: cpp and manual julia installation with this entry.
2. Currently tests are failing on Travis, would be nice to fix.

I made a couple of attempts to fix it in new-travis-infrastructure branch, but since I don't use CLFFT package itself, I leave it to someone with appropriate skills and configured environment.