require 'bugsnag' in rake.rb

Requiring 'bugsnag/rake' in a Rakefile will fail with unknown constant Bugsnag if 'bugsnag' hasn't already been required (e.g. by bundler). Adding this require in rake.rb ensures that developers only need to require 'bugsnag/rake' as per the documentation.
Merge branch 'patch-1' of github.com:danielfone/bugsnag-ruby