Update license headers

[rvantonder]

Closes #2.

grep -l -r "in the \"hack\" directory of this source tree" * | xargs -L 1 -I % bash -c "sed -i'.original' -e 's/in the \"hack\" directory/in the root directory/g' %

[fredemmott]

you've grabbed a very old version of some of these files - if you go look at facebook/hhvm again, you'll find the most recent versions are MIT-licensed

[rvantonder]

Yep--it's worth noting that these versions are consistent with the ones in Pyre, and that's where I took it from. It's worth upstreaming the current files in Hack to Pyre then (and seeing if it breaks anything there). I'm worried that if I just drop in the current Hack files from master that it'll break the interface. Happily accepting PRs; for now it's low priority to check that these are current with Hack. Tracking in #4.