use gem-maven-plugin and exclude unnecessary resources

Refactored pom to use the gem-maven-plugin for downloading gems. The advantage is that gems are only downloaded if not already present (`mvn package` is about 4 times faster than `mvn clean package`).

Additionally, unnecessary resources are excluded from the plugin artifact, thus reducing its size from 4.7M to 719K.

Not sure if the eclipse lifecycle mappings are ok. I use Intellij.