Fix test running in Ruby 2.2.x

The tests will not run under ruby 2.2.x.  Include the test-unit gem to
the Gemfile.  Also add Ruby 2.1.0 and 2.2.0 to the .travis.yml file to
make sure nothing breaks in the future for these versions.

.travis.yml

@@ -4,6 +4,8 @@ rvm:
   - 1.9.2
   - 1.9.3
   - 2.0.0
+  - 2.1.0
+  - 2.2.0
   - jruby-19mode
 
 notifications:

Gemfile

@@ -3,3 +3,4 @@ source "https://rubygems.org"
 gem "shoulda", "~> 2.11.3"
 gem "jeweler", "~> 1.8.7"
 gem "mocha", "~> 0.14.0"
+gem "test-unit", "~> 3.0.8"

Gemfile.lock

@@ -42,11 +42,14 @@ GEM
       multi_json (~> 1.0)
       multi_xml (~> 0.5)
       rack (~> 1.2)
+    power_assert (0.2.3)
     rack (1.5.2)
     rake (10.1.0)
     rdoc (4.0.1)
       json (~> 1.4)
     shoulda (2.11.3)
+    test-unit (3.0.9)
+      power_assert
 
 PLATFORMS
   ruby
@@ -55,3 +58,4 @@ DEPENDENCIES
   jeweler (~> 1.8.7)
   mocha (~> 0.14.0)
   shoulda (~> 2.11.3)
+  test-unit (~> 3.0.8)