Cleanup dependencies in the gemspec.

After adding this gem to a project, I noticed some weirdness around its dependencies:

* It depends on both nokogiri-happymapper and happymapper, but does not appear to use the latter. This has the unfortunate side effect of also making downstream projects pull in an build nokogiri and libxml-ruby.
* It lists crack as a runtime dependency, when it is only used in the project's own tests.
* It lists multiple versions of nokogiri.

This pull request cleans up the gemspec to deal with these issues. I am not familiar with jeweler at all, so I used their Rakefile and Gemfile as guides and could have messed something up.

This is what the dependency portion of the gemspec look like in current release (1.5.10):

```
      s.add_runtime_dependency(%q<rest-client>, ["~> 1.6.0"])
      s.add_runtime_dependency(%q<nokogiri-happymapper>, [">= 0.5.4"])
      s.add_runtime_dependency(%q<builder>, [">= 0"])
      s.add_runtime_dependency(%q<nokogiri>, [">= 1.5.5"])
      s.add_runtime_dependency(%q<crack>, [">= 0"])
      s.add_runtime_dependency(%q<rest-client>, ["~> 1.6.0"])
      s.add_runtime_dependency(%q<happymapper>, [">= 0.3.2"])
      s.add_runtime_dependency(%q<builder>, [">= 0"])
      s.add_runtime_dependency(%q<nokogiri>, [">= 1.4.3"])
      s.add_development_dependency(%q<rspec>, [">= 0"])
      s.add_development_dependency(%q<bundler>, ["~> 1.0.12"])
      s.add_development_dependency(%q<jeweler>, [">= 0"])
      s.add_development_dependency(%q<stale_fish>, ["~> 1.3.0"])
```

This is what the dependencies look like with this patch applied:

```
      s.add_runtime_dependency(%q<rest-client>, ["~> 1.6.0"])
      s.add_runtime_dependency(%q<nokogiri-happymapper>, [">= 0.5.4"])
      s.add_runtime_dependency(%q<builder>, [">= 0"])
      s.add_runtime_dependency(%q<nokogiri>, [">= 1.5.5"])
      s.add_development_dependency(%q<rspec>, [">= 0"])
      s.add_development_dependency(%q<rake>, [">= 0"])
      s.add_development_dependency(%q<jeweler>, [">= 0"])
      s.add_development_dependency(%q<stale_fish>, ["~> 1.3.2"])
      s.add_development_dependency(%q<autotest>, [">= 0"])
      s.add_development_dependency(%q<crack>, [">= 0"])
```