Changed instances of assertTrue(a in b)

Changed instances of assertTrue(a in b) to assertIn(a, b), as the error
messages are more informative.

For example, i was getting:
AssertionError: False is not True

Now, I'm getting:
AssertionError: 'currency' not found in {'token': '123456', 'email':
'seu@email.com'}
Merge pull request #12 from lscasanova/master

Changed instances of assertTrue(a in b)