Lock rest-client to 1.6.x due for Ruby 1.8.7 support
Undo DEFAULT_TIMEOUT = nil, but allow

Since right_api_client must continue to support Ruby 1.8.7, we've locked rest-client to '~> 1.6.7', i.e. 1.6.7 or greater. Setting the :timeout option to nil in 1.6.7 doesn't result in disabling the time. Instead, it sets a 60 second timeout. The nil :timeout setting only works in 1.7.x onward, which doesn't support Ruby 1.8.7
Update CHANGELOG.md
Merge pull request #70 from rightscale/fix_timeout

Fix timeout and lock rest-client to 1.6.x