Bump up the version number

[zhangchicool]

Since the released version 3.2.7a, I have added some new features (mostly clock models), and mentioned the upcoming release version 3.2.8 in some papers. But people who read those papers and compile the current souse code on the develop still get v3.2.7a, and they sometimes get confused. I suggest bump up the version number on the develop branch to "3.2.8", or at least "3.2.8 beta", to distinguish with the released 3.2.7a.

[davidcannatella]

Dear MrBayes maintainers,
I would appreciate you doing this also.
I have read Zhang's paper, and would like to try these analyses on the CIPRES portal (it's not feasible to do them on my laptop).
However, I'm pretty sure CIPRES will not update their version of MrBayes (3.2.7a) if the release number is not updated in some fashion by you.
Best,
Dave

[tiago-simoes]

Agreed. Bump up current GitHub version number to v. 3.2.8 please