[Do not merge] Improve appveyor

[wilzbach]

These versions are pretty old I'd probably just remove support for <2.069

I added a couple of dmd versions to find the lowest working one on Windows. However, it seems that the failure with ldc-beta is a real one:

core.exception.AssertError@source\libasync\test.d(214): Assertion failure
----------------
0x00007FF7FA595CF9 in d_assert
0x00007FF7FA4ED929 in libasync.test.testMultiTimer.__lambda1 at C:\projects\libasync\source\libasync\test.d(214)
0x00007FF7FA4E2A0D in libasync.timer.TimerHandler.opCall at C:\projects\libasync\source\libasync\timer.d(150)
0x00007FF7FA4E03E6 in libasync.windows.EventLoopImpl.onMessage at C:\projects\libasync\source\libasync\windows.d(1825)
0x00007FF7FA4D1C77 in libasync.windows.wndProc at C:\projects\libasync\source\libasync\windows.d(2765)
0x00007FF961E724FD in DispatchMessageW
0x00007FF961E72357 in NotifyWinEvent
0x00007FF7FA4D2890 in libasync.windows.EventLoopImpl.loop at C:\projects\libasync\source\libasync\windows.d(302)
0x00007FF7FA4EA34E in libasync.events.EventLoop.loop at C:\projects\libasync\source\libasync\events.d(302)
0x00007FF7FA4E9E26 in libasync.test.__unittestL13_16 at C:\projects\libasync\source\libasync\test.d(38)
0x00007FF7FA5A321A in int core.runtime.runModuleUnitTests().__foreachbody1(object.ModuleInfo*)
0x00007FF7FA5A670F in int rt.minfo.moduleinfos_apply(scope int delegate(immutable(object.ModuleInfo*))).__foreachbody2(ref rt.sections_ldc.SectionGroup)
0x00007FF7FA5A66BC in int rt.minfo.moduleinfos_apply(scope int delegate(immutable(object.ModuleInfo*)))
0x00007FF7FA5944DC in int object.ModuleInfo.opApply(scope int delegate(object.ModuleInfo*))
0x00007FF7FA5A31E0 in runModuleUnitTests
0x00007FF7FA5A24AF in d_run_main
0x00007FF7FA5687C5 in __entrypoint.main at C:\projects\libasync\__entrypoint.d(8)
0x00007FF7FA5B3DA9 in __scrt_common_main_seh at f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl(253)
0x00007FF9626813D2 in BaseThreadInitThunk
0x00007FF9629F54E4 in RtlUserThreadStart
Program exited with code 1

https://ci.appveyor.com/project/etcimon/libasync/build/job/t4b3mxcpvuq1mfa6

Follow-up to https://github.com/etcimon/libasync/pull/79

[wilzbach]

It looks like 2.070.2 is our winner: