Improve instructions to run tests.

"rake" runs barely any tests:

| porridge@beczulka:~/fog$ bundle exec rake
| Run options: --seed 61633
|
| # Running:
|
| .......
|
| Finished in 0.154545s, 45.2944 runs/s, 45.2944 assertions/s.
|
| 7 runs, 7 assertions, 0 failures, 0 errors, 0 skips
| porridge@beczulka:~/fog$

"rake travis" runs these and the shindo tests
Merge pull request #25 from porridge/master

Improve instructions to run tests.