Setting version to 1.8.0
Setting version to 1.9.0-SNAPSHOT
Remove duplicate RichTreeWalk tests

The RichTreeWalkSpec already has these.
Update to Specs2 v2.1

Version 2 of Specs2 seems to introduce a requirement that the end of an
Example /must/ be convertible to a result, which necessitates a bit of
juggling to avoid errors like these:

could not find implicit value for evidence parameter of type org.specs2.execute.AsResult[Unit]