Remove marginBottom in button style

[luugiathuy]

The reason is that we cannot set marginVertical style for the button as the marginBottom will always override it. The component uses Button have to define both marginBottom and marginTop instead. Thus I think we should let let the one use it define the margin.

Thanks for the library!

[a-morenovalls]

Please, can you revise Travis-CI? Tests are failing, thanks!