Removes trailing slash on bundle_path

There seems to be some kind of conflict with bufkill.vim, triggered by
doing something like:

* "mvim ." in some project
* :Btabedit somegem
* :bd: or :q or anything that kills the window

This only happens with locally bundled gems,
BUNDLE_DISABLE_SHARED_GEMS set, and the BUNDLE_PATH ends with a /, so
chop it off if it's there