Use dirname( __FILE__ ) instead of __DIR__ in trunk loader file

As WordPress minimum PHP required version is 5.2.4 and __DIR__ was introduced in 5.3, we need to use dirname( __FILE__ ) so that people can use the trunk version of BuddyPress if their PHP version is lower than 5.3.

props hnla, boonebgorges

Fixes #5702

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8523 cdf35c40-ae34-48e0-9cc9-0c9da1808c22