don't treat Makefile as an indicator of the project root

Sadly, despite recursive make well-known to be harmful, it is still
in common use, which means that the presence of Makefile in a
directory cannot reliably indicate that that directory is the root
of a project.  Therefore it is safer not to include this in
projectile-project-root-files by default.
Merge pull request #226 from aspiers/fix-recursive-make-projects

don't treat Makefile as an indicator of the project root