Fix #2892: "rake travis" fails with can't modify frozen String

Make an unfrozen copy of RUBY_VERSION so that rubygems version can
strip! it.
Move PrivateIpAddress to the NetworkInterface structure

Only if it exists (in order to associate a public ip).
Merge pull request #2913 from porridge/fix-coveralls

Fix #2892: "rake travis" fails with can't modify frozen String
Merge pull request #2909 from betawaffle/move-private-ip-to-network-interface

AWS: Move PrivateIpAddress to the NetworkInterface structure if it exists