Pin memcached cookbook to version 1.4.0

- berks install is ignoring the Berkshelf.lock file and downloading the
updated versions instead. Memcached 1.5.0 is currently broken due to an
attribute spelling error and fails a chef-client run. Specifying the
version in the Berksfile will for the version to match the version in
the lock file.

- Removing Berksfile.lock from .gitignore

Fixes rcbops-cookbooks/swift-private-cloud#166