HHVM support & compatibility v2

This is lighter solution of #104, without introducing arguments API etc.

ping @milo for review
What about something like:
```sh
export TESTER_PHP_BIN=`case $TRAVIS_PHP_VERSION in hhvm*) echo php;; *) echo php-cgi;; esac`
```
in `before_script` section? And:
```
script: Tester/tester -p $TESTER_PHP_BIN -s tests
```