Switch AppVeyor to Visual Studio 2015
Switch AppVeyor to Visual Studio 2015

Visual Studio 2010 x64 is supposed to enable /Wp64 (detect 64-bit
porting issues) by default but this does not work. Adding /Wp64 will add
a lot of deprecation warnings to the log; but not having /Wp64 has
caused a bug in otherlibs/win32unix/channels.c to go unnoticed.
Therefore, switch to Visual Studio 2015 to workaround the problem.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml