Cleaned up `require`s.

* Use `require_relative` where applicable.
* `require 'rake'` not needed in Rakefile.
* `require 'rubygems'` not needed with Ruby >= 1.9.3.
* Drop Rails 2.3.x/RubyGems incompatibility workaround. (Introduced in 302097a; cf. http://stackoverflow.com/questions/5176782)
Provide entry point matching gem name.

Just as a courtesy for newcomers.
Move version constant into its own file.

No need to load all the dependencies to get at the version number.
Specify required Ruby version.

Settling for 1.9.3 as anything older should really be dead by now.
Made MediaWiki::Gateway#custom_query public.

That was most probably an oversight in 1350fc4 (PR #38 by @shouya).
Formatting cleanup and syntax update.
Specify license in GemSpec.
Group API methods into modules.

As per suggestion by @fhocutt, https://www.mediawiki.org/wiki/API:Client_code/Evaluations/MediaWiki-Gateway. Related to #66.
Remove simplecov dependency.
Remove rr dependency.

Unused since afffb58.
Remove debugger dependency.
Remove atomic dependency.

Unused? Reported as missing in #52 (436cae2).
Swap out builder dependency (via activemodel) for nokogiri.

`nokogiri` also used by `equivalent-xml` and may replace `rexml` in the future.
Swap out activemodel dependency for equivalent-xml.
Remove activesupport dependency.

Use `unicode` or `activesupport` for Unicode support if available.
Remove support files for managing Ruby versions.

We generally default to the latest stable Ruby.
Remove Bundler files.
Swap out jeweler dependency for hen.
Hooking up Travis CI.
lib/media_wiki/utils.rb: Turn into proper module.