only iterate over the versions once and use `groupBy` to sort the dependencies into the version mapping
Merge pull request #9 from jochenberger/optimize-version-mapping

Slight optimization for the version map gathering
Slight reformatting for consistency
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/main/groovy/com/github/benmanes/gradle/versions/updates/DependencyUpdates.groovy