Use nexus 2.6. Fixes #41.

* update java cookbook to ~> 1.12.0
* use java 7 (including a hack to fix java_home and openjdk_packages)
* update version, external_version and checksum for 2.6
Merge pull request #43 from simplymeasured/update-to-2.6

Use nexus 2.6. Fixes #41.