Versions of Windows prior to Windows 8 do not enable TLS 1.2 by default,
though support may exist.  Try to enable TLS 1.2 support explicitly on
connections.

This request may fail if the operating system does not have TLS 1.2
support - the initial release of Vista lacks TLS 1.2 support (though
it is available as a software update) and XP completely lacks TLS 1.2
support.  If this request does fail, the HTTP context is still valid,
and still maintains the original protocol support.  So we ignore the
failure from this operation.

For platforms that do not define `WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_1`
and/or `WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_2`.

Include the constants for `WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_1` and
`WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_2` so that they can be used by mingw.

This updates both the `deps/winhttp` framework (for classic mingw) and
adds the defines for mingw64, which does not use that framework.

travis: use custom libssh2-1-dev package

To avoid pull requests needing to rebase, keep the libssh2-1-dev
package as the development package for libssh2.  Reverting to the
original Debian package structure.

travis: use custom libssh2 package

Use the custom libssh2 package that is a backport of libssh2 1.8.0 to
Ubuntu trusty.

v0.27.0-rc1: version bump