Require 2015 version for MSVC

Require 2015 version for MSVC

C++11 support is best in this compiler in its latest version, which is
an acceptable compromise for this platform.

This is not good for a CUDA build, which won't officially support MSVC
2015 until CUDA 8, which is unlikely to be released before GROMACS
2016. Thus, there is likely to be a few months where CUDA-enabled
GROMACS 2016 cannot be built by a supported MSVC host compiler.

MSVC 2015 adds warnings for illegal implicit narrowing of double to
float, when used in a brace initializer. In some cases
* we intend the interpretation as real, which is now explicit
* we can just use double
* we can suppress the warning
* in some test code, it is more convenient to initialize as double and copy

Change-Id: Ic6b2f9165b6f1aaa3dc59ce05cd6ffb3abe8861c