Start development on 4.0.0.
Add okhttp 1.5.4, Apache Oltu 1.0.0.

- Move maven compiler plugin to 3.1.
(cherry picked from commit 16d7161)
Add versions-maven-plugin 2.1.
Update to retrofit 1.6.0, okhttp 2.0.0-RC2.

- https://github.com/square/retrofit/blob/master/CHANGELOG.md#version-160-2014-06-06
- https://github.com/square/okhttp/blob/master/CHANGELOG.md#version-200-rc2
Fix show summary test.