Update Rake to version 10.1

The Rakefile depends on rdoc so this involves also adding rdoc as a
dependency.
Remove call to :check_dependencies Rake task

The :check_dependencies rake task is not defined in later versions of
Jeweler when using Bundler (see Jeweler commit d5702c3be7fc7ba2e5b).
Therefore, leaving this line would cause an error when we update Jeweler
to version 1.8.
Update Jeweler to version 1.8

Also re-generate gemspec.
Merge pull request #7 from ericmathison/update-rake-and-jeweler

Update Rake and Jeweler