Change test to not compare the implementation with itself.

This has the added benefit of not using next, previous and current on an object
which fails on HHVM.
Change phpunit convertNoticesToExceptions to true.

This makes PHPUnits error handling under hhvm not be broken.
Change travis configuration to not allow HHVM failures.
Merge pull request #251 from wmde/hhvm-workarounds

HHVM workarounds