This reverts commit 127d8b5e.

Closes #479.

If T is a template instantiation and `List<T, kind<T>()>` is the return or
parameter type of a function (notably get, set, init, adopt, and disown
functions) defined out-of-line from its definition, MSVC fails to match
the definition with its declaration. This seems to be another consequence
of poor expression SFINAE / constexpr support.

An easy workaround is to avoid using kind<T>() and instead manually
instantiate Kind_<T>, as in lite mode. When I converted Kind_<T> to use
VoidSfinae, I had thought this solved the problem, but clearly there are
still edge cases.

It turns out that, quite often, the write() will have already completed before the method returns, hence dropping the returned promise won't cause any apparent problem -- until you try to write() a sufficiently large value that it doesn't complete on the first call. Then, you get mysterious bugs.

Indeed, it turns out the HTTP library has such bugs, which are fixed in this change.

This work-around had been removed in 75c7aaee.

However, it turns out that many Debian Jessie systems still use 4.9.2 (not 4.9.4, which is fixed).

I've changed the #if to more specifically target 4.9.x, which should cover all affected versions (in addition to some fixed versions).

or, the world's least significant pull request.

* Announce 0.6 release on blog, and update site.

* When pushing capnproto-next site, include future blog posts.

* Deeper headings.

* Linkify people's names to Github profiles, and finish sentence about JSON.

* Link to exact second in JGC video.

This will break out MinGW, VS2015, and VS2017 builds into separate jobs.
While they will still probably build in serial, we'll at least get better
granularity on test results.

Update outdated comment, and require CMake 3.1, which allows us to take
advantage of target_compile_features.

Projects which use Cap'n Proto require C++11, because they must be able to
compile our headers. Old gcc defaults to C++98, forcing users to manually
specify the -std=gnu++11 flag in their CMake projects. CMake 3.1
introduced the target_compile_features command which removes this
necessity by automatically communicating the C++ standard level
requirement to client projects.

Specifically, if target `kj` requires a C++11 feature publicly, then all
targets which link to `kj` will also require that C++11 feature, and get
the -std=gnu++11 flag for free. If that target is a library, such as
`kj-async`, and `kj-async` links publicly to `kj`, the requirement is also
transitive to all targets which link to `kj-async`.

Note that CMake's default behavior is to request compiler-specific
extensions, such as those provided by the -std=gnu++11 flag. You must
specifically opt out of these extensions. I'm not aware of any way to
propagate this nuance in CMake, but added a check to at least prevent
users from compiling Cap'n Proto without extensions.

With this change, the c++/samples/CMakeLists.txt will build with MinGW
with no change beyond requiring a later version of CMake.

VS2015 only barely compiled ~NullableValue's noexcept specifier
expression. VS2017 can't at all, so we're going with noexcept(false) for
MSVC for now.

Have AppVeyor run the tests.

I can only reproduce this failure on AppVeyor, but the test prints:

capnp\rpc.c++:2886: exception = kj\async-io-win32.c++:399: failed: WSASend(): A request to send or receive data was disallowed because the socket had already been shut down in that direction with a previous shutdown call.

I think this is because WSAESHUTDOWN was considered to be a "bug" exception rather than a "disconnected" exception. It is odd, though, that it doesn't happen when I run locally.