Bump the minimal version required to compile DMD to 2.076.1

[Seb]

Motivation
----------

1) It's required for Walter's work on using -betterC for the 
backend conversion:

https://github.com/dlang/dmd/pull/6907

2) We start to run into random failures lately, e.g.

https://github.com/braddr/d-tester/issues/63
https://github.com/dlang/dmd/pull/7569#issuecomment-356992048

3) There's also WIP to move dmd-cxx to 2.076.1:

https://github.com/dlang/dmd/pull/7595

So if we have to bump the minimal dmd version required to build 
DMD anyways for (1), let's be clear about it and state it.

Anyone objecting?