Bump to 1.6.0-SNAPSHOT

In preparation for a a release that requires IDEA 13.1.
Support and require IDEA 13.1

  - hook into a new protected method to add our prompt handling
  - reflectively set the `ConsoleHistoryModel` field, rather than overrriding
    it. This is a lesser evil than the copy/paste from the base class.
Since IDEA 13.1
Update README
Merge pull request #90 from retronym/topic/1.6

Support and require IDEA 13.1