Fix README to pass pytest doc test.

[Rogdham]

As mentioned in #17, there is a small problem in the readme which makes the Travis builds fail for no real reason.

This PR is to fix it, so that Travis build are meaningful again! Will be great to see by the Travis status if the tests are passing or not.

[keleshev]

Well, it is a bit more complicated then that. The example uses set that means there is no correct oreder:

>>> Schema(set([int, float])).validate(set([5, 7, 8, 'not int or float here']))
Traceback (most recent call last):
...
SchemaError: Or(<type 'float'>, <type 'int'>) did not validate 'not int or float here'
'not int or float here' should be instance of <type 'int'>

[Rogdham]

Yep, that's true. Even if it seems that the set is iterated always in the same order for some reason.

Would you prefer to change using lists instead? I don't think that using a list instead of a set makes any difference at that point in the readme, so it could be a solution.

[keleshev]

Yeah, list or tuple should be alright.

[Rogdham]

I've changed my commit: now it's using tuples.
(List were used in the example just before in the readme, so to have a bit of change I switched to tuples this time)