Updated for i686 arch

Per an IRC party with @simonista, it was found that Switchman expects (2 ** 32).class to be Fixnum. On i686, it's Bignum. Since the [two](https://github.com/instructure/switchman/blob/master/app/models/switchman/shard.rb#L326) [conditionals](https://github.com/instructure/switchman/blob/master/app/models/switchman/shard.rb#L353) don't really seem to care about the subtle bitwise differences between Fixnum and Bignum, and Fixnum is architecture-specific -- and Bignum is going to cast back to integer anyway -- I'm submitting a PR to include Bignum in the aforementioned conditionals.