Follow-ups:

Travis:
* one travis job

Makefile:
* do not try to lint non-existent files

Hook:
* use the jshint executable directly in the hook, so that we never run npm
  install when we just want to commit
* always run both gjslint and jshint, even if one of them fails
* use NO_XFAIL=1 because we already warn about xfailed files
* use LC_ALL=C so that sorting is the same for all platforms

Various:
* move the jshint-specific files to directory build/jshint/
* various other small changes
* update the xfail list