Force filetype plugin on

Resolves #11.
Check for the existence of :Make, not :Makeshift

As suggested by @graywh:
  https://github.com/tpope/vim-rails/pull/258#issuecomment-18103042
Merge pull request #12 from johnsyweb/makeshift-harmony

Check for the existence of :Make, not :Makeshift
Merge branch 'master' of git://github.com/tpope/vim-rake