Resolve app engine map-reduce guava dependency conflict.

Resolves the following app engine dependency conflict.

[ERROR] Failed to execute goal on project karmaexchange: Could not resolve dependencies for project org.karmaexchange:karmaexchange:war:1.0-SNAPSHOT: Failed to collect dependencies for org.karmaexchange:karmaexchange:war:1.0-SNAPSHOT: Could not resolve version conflict among [com.google.appengine.tools:appengine-mapreduce:jar:RELEASE -> com.google.appengine.tools:appengine-gcs-client:jar:[0.3,0.4) -> com.google.guava:guava:jar:[15.0,15.99], com.google.appengine.tools:appengine-mapreduce:jar:RELEASE -> com.google.guava:guava:jar:[14.0,14.99]] -> [Help 1]