[MAESTRO-2821] Updated Rakefile to use maestro-plugin-rake-tasks gem
[MAESTRO-2821] added maestro-plugin-rake-tasks to Gemfile
[MAESTRO-2821] Removed maestro-plugin-rake-tasks from Gemfile

It was causing some issues. The first bundle exec rake would work, but
subsequent invocations would fail.  Best practice is to install the gem in
the global gemset make build process repeatable.