update Riak version to 1.4.1

The current version of Riak is 1.4.1.  This patch updates the brew version from 1.3.1 to 1.4.1

added requirements: Mac OS 10.8 and a 64-bit system
edited the value of RUNNER_BASE_DIR, which Riak needs, but calculates wrong
when it's called through a symlink