Update master to show the latest version of the code to newcomers.

[adrienbernede]

The master branch is outdated because if it is used by someone, updating the branch would like break the CI of users.
That is why it is better to point at version tags, or at list version branches.

I checked the official users mentioned in the readme...

• MFEM uses 2.4
• GlVis uses 2.4
• Laghos uses master
• Remhos uses master

Surprisingly, Laghos and Remhos are both using build-mfem@master. Which means even if we update now, we should allow developers to update their branch before moving the master branch.