Update to Travis CI Trusty environment

[osma]

With the jena3 text index work, I had to downgrade our Travis CI configuration to use the old non-containerized Ubuntu 12.04 environment, because it was the only one where I could get both PHP and Java 8 working (see 93ed7b8). But that configuration is ugly (needs to download and install Java 8 every time), slow and often the Java install breaks, stalling the build.

We should start using the Trusty (Ubuntu 14.04) environment that comes with Java 8. But when I last time tried it didn't come with PHP and the documented method for installing PHP via phpenv didn't work (see travis-ci/travis-ci#4750 and travis-ci/travis-ci#5058).

Apparently this project is using a workaround to install PHP, we could try the same. Or just wait for another relase of the Trusty environment that hopefully comes with PHP support.

We could experiment with different Travis configurations in a branch, if we get this working it could then be merged to master.

Setting it tentatively for 1.4 because it would be nice to have a good CI setup for the release.

[osma]

Started work on this in travis-ci-trusty branch

[osma]

Branch has been merged, most things seem to be working.