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Bugfix: bundler install fails because of bundler:map_bins

capistrano-bundler prefixes the gem command with "bundle exec". That
makes the rbenv:install_bundler command to run like this:

  bundle exec gem install bundler

Which will obviously fail. This happens only if capistrano-bundler is
sourced before capistrano-rbenv.

This commit fixes this problem.
Update changelog
v1.1.0