Turn off the hhvm JIT to speed up the hhvm unit tests.

Source for why the JIT is bad for unit testing: facebook/hhvm#6979

This provides the current HHVM version (3.15.3 as of this PR) and will track with each release (i.e. will be 3.16 when 3.16 is released.

If testing against HHVM LST versions is desired follow this guide. https://docs.travis-ci.com/user/languages/php#HHVM-versions

Should be able to change to container based Trusty after Q1-17 https://blog.travis-ci.com/2016-11-08-trusty-container-public-beta/

this is not supported, yet: https://github.com/facebook/hhvm/issues/2745

currently phpseclib throws an error (about a deprecated function).
errors do not count as failures, so we can't skip them. so for now
7.1 testing is removed again.

See http://docs.travis-ci.com/user/migrating-from-legacy

test (but allow fails) against hhvm and PHP nightly (PHP7) builds
remove phpunit pinning

See here for upstream report:
https://github.com/sebastianbergmann/phpunit/issues/1523

Something is broken on travis-ci.org regarding the max execution time
limit and code coverage generation. I have no clue how to solve this, so
I remove this again for test completion's sake.

Please chime in with ideas at

* https://github.com/travis-ci/travis-ci/issues/1837
* http://stackoverflow.com/q/20972248/172068

not sure if we end up using it