CI config: create build matrix, build on multiple platforms

[aklomp]

Currently, the Travis CI only builds one library on the standard OS and architecture. In order to increase testing coverage, update the Travis config file to include a build matrix with many more OS and architecture combinations.

Note that support for the native Windows MSVC toolchain would require a Cmake-based build configuration, which we don't have (yet), so that platform will initially not be supported.