Updated exists? to exist? to prevent deprecated call messages.

When saving a file using Ruby 2.1.0 you would get the following console error/warning:

/root/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rmate-1.5.5/bin/rmate:108: warning: File.exists? is a deprecated name, use File.exist? instead
/root/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rmate-1.5.5/bin/rmate:109: warning: File.exists? is a deprecated name, use File.exist? instead