Tests: Clean up $wp_version before deciding whether to use 3.9+ go_to()

version_compare() was getting hung up on some WP version numbering schemas,
such as the addition of '-src' when running from a develop.svn.wordpress.org
checkout (like in the case of travis-ci).

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8396 cdf35c40-ae34-48e0-9cc9-0c9da1808c22