Use run_context.loaded_recipe? to check for ruby_build

This allows you to use `include_recipe("ruby_build")` instead of having
to add it to the run_list.

Fixes #31 and #35.
typo
Add a definition_file attribute to the rbenv_ruby resource.
This prevents the recipe from continually trying to build a custom
ruby when passed a build file name instead of a built-in definition
Updated gem resource to allow either String or Hash options, as the Chef Rubygems provider allows both
Update Opscode documentation links

Most of the wiki.opscode.com links redirect, but not to the right place. This updates the links to the new documentation site.
Update Test Kitchen gem version and base box defaults.
Fix README title and a couple of dated tidbits.
Merge pull request #37 from zozi/fix_ruby_build_missing

Use run_context.loaded_recipe? to check for ruby_build
Merge pull request #45 from aderyabin/master

Typo in Readme
Merge pull request #46 from jf647/prefix_path

Add a definition_file attribute to the rbenv_ruby resource.
Merge pull request #53 from sectioneight/patch-1

Update Opscode documentation links
Update .kitchen.yml to provsionless vagrant base boxes.
Update suite test Ruby version.
NO MORE RDOC WHEN COMPILING RUBIES IN TEST!

Seriously, this burns up so much time in the build process. Some hints
taken from sstephenson/ruby-build#156.
Drop default "global" version in rbenv_script provider.

This will lead to errors if rbenv_script is invoked before any rbenv
Ruby versions are installed (and a global Ruby version is set).

References #33
References #43
Merge pull request #48 from swieton/master

Allow String or Hash for ruby gem options