Add missing dependency on commons-lang 2.6

Without this dependency, use of os-package-plugin gave the following
error:

Execution failed for task ':buildDeb'.
> Could not call SystemPackagingTask.copy() on task ':buildDeb'

due to java.lang.NoClassDefFoundError: org.apache.commons.lang.time.DateFormatUtils
Upgrade to commons-lang3 3.1

3.1 has been out since November 2011, so it is time to upgrade from
2.x.

Ref: http://commons.apache.org/proper/commons-lang/article3_0.html
Merge pull request #1 from cloudfoundry/master

commons-lang dependency improvements