force usage of Groovy 1.8.9 because of http://jira.codehaus.org/browse/G...

Groovy 1.8.9 seems to have a significant bug fixed regarding memory consumption, http://jira.codehaus.org/browse/GROOVY-5187. This PR adds a explicit dependency to Groovy 1.8.9 overriding the transitive dependency to Groovy 1.8.4 from gremlin-groovy-1.5.