Removes duplicated inclusion of Rake::DSL

[grzuy]

Rake::DSL is already included in Rake::TestCase

[grzuy]

Apparently test build failed only for ruby 2.1, because bundle install command was installing rdoc 5.x previously for all builds, but installed rdoc 6.x for the first time with this PR (despite rdoc 6.x released 1 2 months ago), which is no longer compatible with ruby 2.1

Fetching rdoc 6.0.1
Installing rdoc 6.0.1
Gem::RuntimeRequirementNotMetError: rdoc requires Ruby version >= 2.2.2. The
current ruby version is 2.1.0.
An error occurred while installing rdoc (6.0.1), and Bundler cannot continue.
Make sure that `gem install rdoc -v '6.0.1'` succeeds before bundling.

[grzuy]

Reported issue here #255

[hsbt]

@gazay Can you rebase from master?

[grzuy]

@hsbt Rebased

[yuki24]

I'm merging this PR since this seems like a safe change. Thanks @grzuy!

[grzuy]

Thank you!