Fix for Debian Ruby 1.9

I know Discourse requires Ruby 1.9.3 but I tried running Discourse with the Debian Squeeze ruby1.9 package which is 1.9.2. It works fine so far except when you use `__method__` in `lib/discourse_redis.rb`.

I can't find why you wouldn't use the variable `m` here instead of `__method__`, even on Ruby 1.9.3, so I suggest this change.