Updating twitter gem, we were using a prelease version

This PR fix the build problem for the twitter input format.
the gem http-0.6.0.pre is trying to use a new version of the http_parser.

Since we were using a pre release version of the twitter gem,
I think it make sense to update it to a stable/official version.

You can look at the changelog: https://github.com/sferik/twitter/blob/master/CHANGELOG.md


```
Installing rumbster 1.1.1
Installing maven-tools 1.0.5
Installing ruby-maven 3.1.1.0.8
Using logstash 2.0.0.dev from source at /home/jenkins/workspace/logstash_input_twitter_commit/logstash
Your bundle is complete!
Gems in the group development were not installed.
It was installed into ./vendor/bundle
validating ../logstash-input-twitter/logstash-input-twitter-0.1.0.gem >= 0
Valid logstash plugin. Continuing...
Gem::ImpossibleDependenciesError: http-0.6.0.pre requires http_parser.rb (~> 0.6.0) but it conflicted:
  Activated http_parser.rb-0.6.0 instead of (~> 0.5.0) via:
    twitter-5.0.0.rc.1, logstash-input-twitter-0.1.0

```