lock file prevents install with --version of plugins that are needed by others

In a release, the .lock file will contain the reference to a plugin's locked version even after being uninstalled IF other plugins depend on it. Example:

```
/tmp % tar -zxf logstash-2.0.0.dev.tar.gz
/tmp % cd logstash-2.0.0.dev
/tmp/logstash-2.0.0.dev % grep "logstash-codec-json .*0" Gemfile.jruby-1.9.lock
logstash-codec-json (0.1.7)
/tmp/logstash-2.0.0.dev % bin/plugin uninstall logstash-codec-json
Uninstalling logstash-codec-json
/private/tmp/logstash-2.0.0.dev/vendor/local_gems/fccdd8d6/logstash-core-2.0.0.dev-java/lib/logstash/version.rb:3 warning: already initialized constant LOGSTASH_VERSION
/tmp/logstash-2.0.0.dev % grep "logstash-codec-json .*0" Gemfile.jruby-1.9.lock
logstash-codec-json (0.1.7)
/tmp/logstash-2.0.0.dev % bin/plugin install --version="0.1.6" logstash-codec-json
Validating logstash-codec-json-0.1.6
Installing logstash-codec-json
Plugin not found, aborting
ERROR: Installation Aborted, message: You have requested:
logstash-codec-json = 0.1.6

The bundle currently has logstash-codec-json locked at 0.1.7.
Try running `bundle update logstash-codec-json`
/tmp/logstash-2.0.0.dev %
```