Add tests

This is an attempt to add some tests in jumpup. Jumpup is very simple, it just runs each rake task defined in the `INTEGRATION_TASKS` constant, but even being simple, I guess this block the people to fix some issues here because the lack of tests.

Jumpup is very tied to rake, so in the tests I just load a rake file fixture, and programatically call `Rake::Task["integrate"].invoke`. Please give me know if this tests make sense.

//cc @maurogeorge @fgrehm @thiagogabriel @TiuTalk 
This is some changes needed to make the tests pass, in that case the output redirection. 
When running commands with backticks (\`echo hello\`) just the stdout is outputted, because this in failing scenarios the stderr is swallowed, and I cannot compare in the tests.