(maint) Remove dependency on rapidjson from lib.

Recently we removed rapidjson as an external project dependency and
instead have unpacked it in the vendor directory.

As a result, the lib project should no longer have a dependency on
rapidjson, which is header-only.

This fixes a warning in CMake 3.0.
Merge pull request #60 from peterhuene/cmake

(maint) Remove dependency on rapidjson from lib.