Limit method_missing to only handle known commands

Redis-Namespace has had a *long* history of issues rising from the blind
passing of unknown commands to the underlying Redis Client (see almost
every single issue in the [tracker][]). This patch is a breaking change
that stops that behaviour, instead raising a NoMethodError when handling
a command that is unknown.

TODO before 2.x release:
 - [ ] Add support & specs for Command Aliases
 - [ ] Define all methods to avoid method_missing altogether
       (via `Module#define_method`?)

[tracker]: https://github.com/resque/redis-namespace/issues?state=closed

Pairing session with @chelseakomlo and @yaauie.
Remove support for command aliases

Support for command aliases was dropped in redis-rb 2.0, and we have
been requiring redis-rb 3.x for a while now, so the referenced code
is orphaned and *never* used.
fix build for 1.8.7
Using super() to route through #method_missing is awkward.
minor cleanup to #respond_to_missing? for clarity
ah, 1.8.x didn't have a respond_to_mising? super method...
Merge branch 'fix-rubinius-build-on-travis' into 2.0
Merge pull request #73 from resque/2.0

Only wrap known commands, eliminating the root cause of the
vast majority of past issues in this project.