Add dependency to 'clean-cache' ant target

I was previously having problems [1] which resulted in a build failure from a fresh SVN checkout. The problem was due to one other project that had already written to the same file, but with a different revision. This means that Gora uses the shared default root directory which is ~/.ivy2/, which in turn results in the build failure.

[1] http://mail-archives.apache.org/mod_mbox/incubator-gora-dev/201108.mbox/browser