MNT: Bump to latest base version. Relax pinning on compile time dependencies

Update to the latest version of the fsl/base project, and tweak dependencies.

I've had various problems with pinning various packages, either package conflicts raised by conda build (e.g.), or compile-time conflicts caused by conflicting packages being installed (e.g.). These can all be resolved by relaxing the version constraints on dependencies - typically, only pinning against the major/minor version numbers will suffice.

As outlined in the conda-forge documentation, the current level of ABI compatibilty for a range of C/C++ packages is hard-coded in the conda-forge-pinning project. I think we will be safe, as long as we don't pin core dependencies any stricter than what is specified in that file.