Add notice to s/Plugin/Bundle for old Vundle

Summary:
... in installation instructions. Thanks to #17 for updating Vundle's command to
the latest. This just clarifies a bit in case of confused old Vundle users.

Test Plan: refreshed browser.

Reviewers: xymostech, spicyj

Reviewed By: spicyj

Differential Revision: http://phabricator.benalpert.com/D205