Added initial version of a test case for Tumblr

I found tumblr.rb not an easy module to test. I have started with the most simple data check from a mocked JSON payload and was able to cover the basics of JekyllImport::Tumblr.post_to_hash. I can go just a little bit further without refactoring the module, so I thought I should start a pull request to gather feedbacks.

Oh, and I had to bump rake version up because I was hitting the issue mentioned [here](https://github.com/jimweirich/rake/issues/198).