Update openjdk.yml

Just discovered http://support.run.pivotal.io/entries/57707216-Support-Java-8 without which you run into 
2014-07-12T11:50:32.57-0400 [STG]     OUT -----> Downloading Spring Auto-reconfiguration 0.8.9 from http://download.run.pivotal.io/auto-reconfiguration/auto-reconfiguration-0.8.9.jar (0.0s)
2014-07-12T11:50:33.05-0400 [STG]     ERR E, [2014-07-12T15:50:33.050087 #131] ERROR -- /tmp/buildpacks/java-buildpack/lib/java_buildpack/buildpack.rb:49:in `rescue in drive_buildpack_with_logger': Release failed with exception #<RuntimeError: 'permgen' is not a valid memory size>