Added Buildr 'Buildfile' and 'buildfile'

The Java build system 'Buildr' uses a Ruby build file, similar to Rake.  This change adds syntax highlighting for `Buildfile` and `buildfile`, the primary name(s) of the Buildr file.

All other Buildr filename options (as listed at https://github.com/apache/buildr/blob/master/lib/buildr/core/application.rb#L110) are already covered by other things here.