[Development] Let's please drop MSVC 2013 for 5.10

[Shawn Rutledge]

> On 16 Jun 2017, at 11:14, André Pönitz <apoenitz at t-online.de> wrote:
> 
> On Thu, Jun 15, 2017 at 04:07:03PM -0700, Thiago Macieira wrote:
>> character set. Without the /utf-8 option added in MSVC 2015 Update 2, the 
>> conversion fails and the compiler prints:
>> 
>> warning C4566: character represented by universal-character-name '\u0431' 
>> cannot be represented in the current code page (1252)
>> 
>> [ok, not a C++11 issue per se, but we still want to use Unicode source code]
> 
> There was never *consensus* on using something outside plain ASCII in
> Qt sources.

The backslash, u, and the digits are all ASCII characters.  This time we are not talking about utf-8 in source code.