adds php 5.6 and HHVM to .travis.yml
disables mongo.so installation (we skip mongo tests anyway)
misc fixes for HHVM
 - changes algos to constants (integers) for php 5.4-5.6
 - casts apache_request_headers() to array
 - fixes array_walk strangeness
Merge pull request #355 from bshaffer/issue-352-add-HHVM-and-php56

Issue 352 add hhvm and php56