bundle update; Commit updated Gemfile.lock

Attempting to deal with the following Bundler issue:

    Fetching gem metadata from https://rubygems.org/........
    Fetching additional metadata from https://rubygems.org/..
    Resolving dependencies...
    You have requested:
      sprockets = 2.2.2.backport2

    The bundle currently has sprockets locked at 2.2.2.
    Try running `bundle update sprockets`

Signed-off-by: Alex Coles <alex@alexbcoles.com>