Add dependency 'test-unit'

Ruby2.2 doesn't contain it as default.
Allow Ruby 2.2 styled error message

``` diff
-No such file or directory - features. Please create a features directory to get started. (Errno::ENOENT)
+No such file or directory @ rb_sysopen - features. Please create a features directory to get started. (Errno::ENOENT)
```
Specify method call to default argument

The behavor of method as default argument has been changed in Ruby2.2.
see: https://bugs.ruby-lang.org/issues/10314
Add explicity ruby 2.2 support
Merge pull request #774 from tricknotes/support-ruby2.2-for-1.3.x

Support ruby 2.2 for 1.3.x