Fix rspec invokation in Rakefile due to changes in rspec 3.0.0 + fixes for deprecation warnings

In `rspec` 3.0.0 the support for the `nested`-format was removed. If I understand that correctly one should use `documentation` instead. 

Please see https://github.com/rspec/rspec-core/blob/26698e46d312802483cee456610508f92a3dde56/Changelog.md#300beta2--2014-02-17

    Remove support for specifying the documentation formatter using 's',
    'n', 'spec' or 'nested'. (Jon Rowe)

Along with this fix I also fixed some deprecation warnings:
* be_true -> be_truthy
* be_false -> be_falsey
* stub_const