Release 9 of the FreeOrionSDK

v9 - 2018-05-23

Replace FreeOrionSDK Apple Disk Image release with a bzip2 compressed
tarball and remove bootstrap script. The FreeOrion build system now
implements the automatic download and unpacking of the SDK so the
additional dmg image wrapping only adds needless complexity.
Add FreeOrionSDK release for Visual Studio 2017 support, using the v141_xp
toolset.
Update zlib version to 1.2.11
Update libpng version to 1.6.34
Update boost version to 1.66
Disabled Boost library:
- fiber
Use simpler boost shared libary naming scheme on Windows
Update GLEW version to 2.1.0
Update SDL2 version to 2.0.8
Update libopenal-soft version to 1.18.2
Update freetype2 version to 2.9
Build freetype2 as shared library on Windows
Remove freetype2 shared library suffix on Windows
Use upstream conventions for freetype2 install header path
Use upstream conventions for freetype2 ft2build.h install path.
Build freetype2 as shared library.
Update Python version to 2.7.14
Add ccache to Ubuntu Dockerfile to utilize build cache on TravisCI.
Pin Docker image to a stable and more recent base image
(docker.io/ubuntu:xenial-20180417).
Don't install package documentation into Docker image
Move suppression of recommends installed packages into config file for Docker
image.
Don't install static libraries into Docker image
Set Debian frontend to noninteractive to avoid warnings during apt-get usage.