Make the BrowserEnvironment have the gaze cursor enabled on mobile by default.

[capnmidnight]

In order for the gaze cursor to work on mobile devices, the person implementing a script must set a configuration parameter named useGaze to the isMobile flag when setting up their BrowserEnvironment

All that needs to be done is to change the BrowserEnvironment's DEFAULTS object at the end of the file. It current has useGaze set to false. It should be set to isMobile. Also, look for the line import isiOS from "../flags/isiOS"; and add a line below it to import the isMobile flag from the same flags directory.

[hur]

Submitted a pull request (Submitting straight to master is ok for minor bugfixes according to submission guidelines?)

I'm new to git so I hope you could point out if I did something wrong.

A specific question: should CONTRIBUTORS.MD not be updated for something so small as this? I did it just in case.

[capnmidnight]

Thanks for the PR!