Fix test suite

Fixes

```shell
rake aborted!
ERROR: 'rake/rdoctask' is obsolete and no longer supported. Use 'rdoc/task' (available in RDoc 2.4.2+) instead.
/.../tanker/Rakefile:44:in `<top (required)>'
(See full trace by running task with --trace)
```

which will also fix Travis build. 

In addition also fixes a couple deprecation warning coming from RSpec.