sbt-trickle 0.2.3

This adds trickleCheckVersion. This task doesn't use the metadata repository
in any way. Instead, given a list of dependencies and versions to check, it
verifies that they all match what the repository current dependencies are.

The need for this became apparent when we saw trickle trying to update
repositories that had already been updated, and it turned out that the version
for one particular dependency was hard coded and, thus, ignoring the versions
file.

This allows builds to introduce a sanity check before actually creating a
pull request for an update, and signal unmanaged dependency versions.