Support specifying different ruby versions for binaries

[nisanthchunduru]

Support prefixing different binaries with different ruby versions

For example

set :rvm_ruby_version, 'ruby-2.0.0-p481'
# eye, a process manager, runs with ruby-2.2.0
# clockwork can run with the configured ruby version ruby-2.0.0-p481
set :rvm_map_bins, fetch(:rvm_map_bins, []).push(['eye', 'ruby-2.2.0'], 'clockwork')

[nisanthchunduru]

@kirs Anything preventing this from being merged? Thanks.