Appium tests in Travis CI

[hypery2k]

Would be great to have some appium tests within Serenity to ensure that the integration is working.
I've already added some Android test in #415, but would be great to have iOS tests, too.
To do this, Travis CI needs to be run in an OS X environment. That would also make the Android tests run faster, but would run all the Travis CI build in an OS X environment.

What do you think about that?

[YamStranger]

@hypery2k great idea

Thanks!