Bump the minimal version required to compile DMD to 2.076.1

[Jonathan Marler]

On Friday, 12 January 2018 at 16:50:21 UTC, Joakim wrote:
> On Friday, 12 January 2018 at 16:13:39 UTC, Seb wrote:
>> 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
>
> Seem like issues with 32-bit OS X instead.
>

It's actually a bug in dmd 2.068.2.  I was able to reproduce the 
segfault on my macbook.  If I went to the next version of dmd, 
namely, 2.069.0 then the issue went away.  I was also able to get 
a stack trace:
---
Obj::term(char const*) + 2285
obj_end(Library*, File*) + 37
tryMain(unsigned long, char const**) + 12066
_start + 203
start + 33
---

The `+ <num> ` is referring to the code offset in bytes from the 
beginning of the function, so the actual line of code where the 
invalid access occurs is somewhere inside Obj::term.  
Unfortunately, that function is quite large (over 700 lines).

If we want to continue supporting dmd 2.068.2, I would recommend 
patching this bug.  I don't know how long we have been using this 
specific version of dmd 2.068.2 on the autotester, does anyone 
know?