Support for --platform flag to install command and added new platform ruboto(dalvik)

I've continued this pull request https://github.com/bundler/bundler/pull/1164 and I believe I have solved the 
problems it had. Also I added support for dalvik platform https://github.com/rubygems/rubygems/pull/539 .
I have used ruboto as the name conversion for the dalvik platform.
Since dalvik version number must be forward compatible, I also created a new comparison method between 
platforms that is only used with dalvik. So for example a `dalvik_10` gem is allowed to be installed when you use
`bundle install --platform=ruboto_5`.