Correct dependencies in README

Longer/lyude-overlay/#1 pointed out that corebird needs libgee-0.8 to build, and that it won't build correctly with anything older then libsoup 2.42. Not having libgee installed when trying to build corebird results in cmake throwing up an error, and trying to compile with older versions of libsoup (I tried with 2.40.3) results in a linker error.