Add memcached support

Added memcached support to auto-configure and cf-runtime gems.
Uses the [dalli](https://github.com/mperham/dalli) gem because it supports SASL and is [recommended](https://devcenter.heroku.com/articles/memcache#gem-installation) by heroku.

Bumped the cfoundry gem to a more recent version.

Added rake task for using cfruntime from source in cfruntime-tests. This to test unreleased changes.

This is a copy of https://github.com/cloudfoundry/vcap-ruby/pull/1