Added HHVM to travis.yml

[Nyholm]

No description provided.

[ruflin]

Good idea. Can you update the changes.txt?

[ruflin]

BTW: There seems to be a problem with the hhvm builds:

The `phpize' command exists in these Ruby versions:
  5.2
  5.2.17
  5.3
  5.3.27
  5.3.3
  5.4
  5.4.29
  5.5
  5.5.13
  5.6
  5.6.0beta4

https://travis-ci.org/ruflin/Elastica/jobs/29490619

[Nyholm]

We don't need to install memcache on HHVM. It is already installed.
I've updated the PR.

[ruflin]

Please also add the pull request id

[ruflin]

There seem to be problem with one of the Proxy tests:

https://travis-ci.org/ruflin/Elastica/jobs/29506641

[Nyholm]

Since the tests pass on PHP 5.6 I removed it from the allowed to fail.

The tests are now running on HHVM. This was the intention of this PR. Some tests fail though... I do not know how to fix those. Someone with more knowledge of this lib will make sure they pass.

[ruflin]

As it is only 1 test that fails, I would suggest we add a skip to this test in case it is hhvm.

I had 5.6 still on the "can fail" list as the release is not final. But I agree to move it as the release candidates are out.

[ruflin]

Merged in through #778