Use #name instead of #to_s when getting the name of constants.

The only constants that the Inspector should be looking at are of type Module or Class. Both of these built-in types respond to #name, but some gems (such as LibXML) create Modules in such a way that they implement #to_s with one (required) argument, whereas here we only call it with 0 arguments.

In most cases #to_s will be an alias for #name, and #name is really what we want anyway. Aside from that, many core classes and core functionality of Ruby will break if a gem maintainer chooses not to implement #name, whereas not implementing the zero-argument #to_s message appears to have no ill effects outside of causing ruby-lint to raise an exception when generating definitions for these gems.

Fixes [#133](https://github.com/YorickPeterse/ruby-lint/issues/133).
Merge pull request #136 from allquixotic/fix-issue-133

Use #name instead of #to_s when getting the name of constants.