Gem::SourceIndex is removed

Gem::SourceIndex is removed in rubygems 2.0.0.preview2.2

Fix error below:

```
Error: test_use_gem_source_index_dirs(GemOpenTest)
  NameError: uninitialized constant Gem::SourceIndex
/Users/sane/work/ruby-study/gem-open/test/gem_open_test.rb:22:in `test_use_gem_source_index_dirs'
```