add RandomVerticalFlip transform

[alykhantejani]

Closes #257.

I've currently done the same as RandomHorizontalFlip which uses a fixed probability of flipping of 50% and I've only used this probability in the tests.

This is easy to extend to have the probability of flipping be different - but not sure if this is a valuable feature?

Also note, the tests for these require scipy which, at current, is an optional dependency for torchvision. @soumith Do we have scipy on the build box/ is there a way to have it installed for test runs?

[fmassa]

I think this looks good!
We have already added support for scipy in the test machines, so I think this is fine.
About adding support for different flip probabilities, I think this can be added in a later PR if needed.

[alykhantejani]

@fmassa shall we merge this now, or after #240 ?

[alykhantejani]

@fmassa Having a look at the travis job config + log it looks like scipy isn't installed but should be easy to add. The test output also confirms that the test was skipped.