Update version of jsexec required.

Conflicts:
	jshint.gemspec
Try to factor out the rake task logic into a separate executable script.
Specify a minimum version instead.
I mean, greater than 1.4, so that old Rails projects won't break.
Merge remote-tracking branch 'remotes/origin/lowjoel-fix-execjs-version'
Merge remote-tracking branch 'remotes/origin/lowjoel-standalone-script'