Update versions for CI

Resque has a lot of users on older Rubies, so it's important that we
support them. But we have pretty much every Ruby version ever, and
that's too much.

I'm going to introduce "tiers" of support:

Tier 1: blocking
----------------

These are tested in CI, and we won't cut a release without it passing.

This list is:

* 1.8.7
* 1.9.3
* 2.3.0

... also known as the last release in each version line.

Tier 2: community
-----------------

We consider these Ruby versions supported, but won't test in CI.
Patches to fix breakage on these versions are very welcome, but we
probably won't actively prioritize efforts ourselves.

This list is:

* 1.9.2
* 2.0.0
* 2.1.0
* 2.2.0

Unsupported
-----------

We don't support these versions at all. If you can submit a fix for
breakage on these versions, we'll take it if it doesn't break newer
versions, but won't guarnatee that they won't end up broken in the
future.

This list is:

* 1.9.1
* 1.9.0
* All versions before 1.8.7

.travis.yml

@@ -1,14 +1,10 @@
 language: ruby
 rvm:
   - 1.9.3
-  - 2.0
-  - 2.1
   - 2.2
   - 2.3.0
 sudo: false
 matrix:
   include:
     - rvm: 1.8.7
       gemfile: Gemfiles/Gemfile.1.8.7
-    - rvm: 1.9.2
-      gemfile: Gemfiles/Gemfile.1.9.2