remove support for building with older Visual Studio versions

VS2013 Community Edition can be used for development of open-source
projects for free. In addition, the builds for the older versions
are not being tested (e.g. for compilation errors) so it makes sense
to standardize on VS2013.
remove instructions for building on OS X 10.6

The current releases are built on OS X 10.7 and hence these remain
untested -- it is better to document a process which is being used
regularly and is repeatable.