Test more toolsets

[Manu343726]

@pfultz2 asked me on twitter about MinGW support on linux. Add this toolset to the Travis CI build matrix.

[pfultz2]

I believe the MinGW compiler on travis is probably 4.6. I haven't found an update debian package yet for ubuntu 12.04. I thinking a better approach would be to build mingw 4.9 with mxe, and then download the binaries during preinstall. The nice thing about this approach is that mxe already provides the necessary cmake toolchain file.