Enable heuristics.rb

This seems to be a nice addition. It passes the tests and was disabled without explanation in c8761d39d759610f21d4dc207f1191567e1472c7. Current commits seem to want to use it and not realize it's off, e.g., bb2afbb03dbce9b39989edd876cdb41fe482690b. In particular, *I* want to use it to detect Hack vs PHP in the `.php` file extension; the classifier has basically no hope of ever getting this right with an acceptable false positive rate, but a simple regex will do the job.

So, enable heuristics.rb, in preparation for adding a PHP vs Hack heuristic in a later diff.
Add detection for Hack files with ".php" file extension

Based on top of PR#1447. Adds a simple heuristic check for Hack files vs PHP files (`<?hh` vs other `<?`).

Tested by verifying that the Hack example site was detected as 100% Hack and that Laravel was detected as 100% PHP. (Without the heuristic, Laravel gets detected as about 50% Hack, just by randomness in the classifier since PHP and Hack are very hard to distinguish unless you actually parse the file and look for specific language features.)