Replace therubyracer with mini_racer

therubyracer doesn't appear to be under active development

I didn't need this for PRs that I was doing, but a `bundle install`
failed because of it. I know I can specify the groups as a workaround
but replacing this seems like it might reduce a small barrier to
contributing

Gemfile

@@ -43,7 +43,7 @@ group :development do
 end
 
 group :assets do
-  gem 'therubyracer'
+  gem 'mini_racer'
   gem 'uglifier'
 end