Default branch changing to develop

[sebrowne]

In order to facilitate better a smoother experience for developers and users, Trilinos will be changing the default repository branch to be develop. This means that when a new clone is performed, that branch will be checked out, and all new Pull Requests will target this branch by default.

The master branch will still exist as our release-candidate branch for future releases.

If you want to use a released version of the code, you can download it from https://github.com/trilinos/Trilinos/releases

This change will take effect at the end of July.