Fixes large-string expansion in JSObfu.
Use implicit return for assignment.
Use tiny var names by default.
Use actual vars so that jsobfu can randomize.
Kick rkelly out to a gem, add rkelly-remixed.

rkelly-remixed is a faster fork of rkelly that is more frequently updated
nowadays. With the new gem, jsobfu obfuscates os.js about twice as fast on
my dev environment.
Tweak Gemfile comment.
Re-add the #random_string(len) method to pass specs.
Land #3224 - Fixes large-string expansion in JSObfu
Land #3227 - Remove bundled rkelly, to Gemfile