Check filetype before sending Rake autocmd

I ran into a obscure bug using vim-rake in a go project that used a Rakefile to preform various tasks. I have my own vim script that overrides `:A` to alternate between tests and implementation for go files but the `A` command was being overridden by vim-rake because I had a Rakefile in my root.

I'm not sure if this is the best way to fix it but this did work for me.