Bump the minimal version required to compile DMD to 2.076.1

[Seb]

On Friday, 12 January 2018 at 16:50:21 UTC, Joakim wrote:
> On Friday, 12 January 2018 at 16:13:39 UTC, Seb wrote:
>> Motivation
>> ----------
>>
>> 1) It's required for Walter's work on using -betterC for the 
>> backend conversion:
>>
>> https://github.com/dlang/dmd/pull/6907
>
> Not really, he just wants to dogfood betterC on the dmd backend 
> when it's ported to D.

Yeah and that requires:

-mv switch
- removal of the generated assert helper functions

Both are features which aren't present in 2.068
The entire reason why the PR wasn't merged yet is because the 
auto-tester hasn't been updated (and no one hacked these features 
into the gdc Perl wrapper).