Upgrade JSHint to 2.5.4

Previously we vendored + patched a copy of `JSHint` each time we needed to upgrade. This commit takes a plain copy of `JSHint` and adapts `PreCommit` to use `JSHint` out of the box.

This should make future `JSHint` upgrades much easier :)

@shajith @mpapis @mwerner 

Closes #157 
maybe memoize as a `context` method?