Use bundler as recommended for gems

This means that you can run 'bundle exec' in this directory to use the
current development version of the gem.

http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Use cause from ruby 2.1 [https://bugs.ruby-lang.org/issues/8257]
Ignore Gemfile.lock
Remove jeweler
Update CHANGELOG
Version bump to 1.8.0
Merge branch 'master' of https://github.com/bugsnag/bugsnag-ruby