Conflicting jar versions can cause plugins not to work properly

Raised becuase of a conflict with lucene versions between elasticsearch and neo4j, the test raised the situation where having two conflicting versions cause the plugins not to register.

To reproduce this issue:

1.- Install logstash-input-neo4j.
2.- Install logstash-output-elasticsearch.
3.- Add development dependencies.
4.- Run the plugin test.

then you will get an error like:

```
Failures:

1) outputs/elasticsearch registration should register
Failure/Error: expect {output.register}.to_not raise_error
expected no Exception, got java.lang.NoSuchFieldError: LUCENE_3_6 with backtrace:
# org.elasticsearch.Version.(org/elasticsearch/Version.java:43)
# org.elasticsearch.client.transport.TransportClient.(org/elasticsearch/client/transport/TransportClient.java:168)
# org.elasticsearch.client.transport.TransportClient.(org/elasticsearch/client/transport/TransportClient.java:123)
# java.lang.reflect.Constructor.newInstance(java/lang/reflect/Constructor.java:526)
# RUBY.(root)(./rakelib/test.rake:38)
# ./rakelib/test.rake:38:in `(root)'

Finished in 0.967 seconds
21 examples, 1 failure
```

but if for example you install the logstash-output-neo4j and the logstash-output-elasticsearch, the neo4j plugin output an error as it can has a conflict with the lucene version.

lucene jars being used are:

```
.//bundle/jruby/1.9/gems/logstash-output-elasticsearch-0.2.4-java/vendor/jar-dependencies/runtime-jars/lucene-core-4.10.4.jar
.//bundle/jruby/1.9/gems/neo4j-community-2.0.1/lib/neo4j-community/jars/lucene-core-3.6.2.jar
```

This happen now with Lucene, but I guess it can happen with other dependencies alike in the feature too.