On Ubuntu 16.04 (Xenial), we have to build mbedTLS from source because
the version available from apt, mbedTLS 2.2.1, does not work with the
RTMPS implementation in OBS, which requires mbedTLS 2.7 at minimum. This
is due to the use of the function mbedtls_ssl_conf_dh_param_bin, which
was introduced in mbedTLS 2.7.

Build on Linux (Ubuntu 16.04) with AppVeyor. Disable PowerShell on
Linux. Ubuntu 16.04's dependencies are different from the existing
Ubuntu 14.04 dependencies, so a separate script for installing those
dependencies has been added.

This reverts commit de32d89f.

Fixes an issue where Linux CI builds fail because FFmpeg fails to
compile. Using FFmpeg git master in CI builds was obviously not the
smartest idea because changes in FFmpeg can break things unexpectedly.
Sorry about that.

The reason for the original commit was that OBS failed to compile
with FFmpeg 2.4. That has been fixed since.

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