Upgrade rake to version 12.0.0

[depfu-test[bot]]

Hello,

We've upgraded a dependency and all tests pass. \o/

gem name	version specification	new version
rake	~> 11.1	12.0.0

You should probably take a good look at this before merging this pull request, of course.

These are the last commits on the dependency:

• bump version to 12.0.0
• remove rubocop, because it needs Ruby 2.0 later. I will drop Ruby 1.9 and JRuby 1.7 at 2017/3
• History
• cleanup
• use assert_nil instead of "assert_equal nil"

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This is an automated pull request by Depfu. You're getting it because someone configured Depfu to automatically update dependencies on this project.