change to libjson-c >=0.12

- Update to libjson-c >=0.12 to get rid of the deprecated functions (see #72)
(probably libjson-c >=0.10 is sufficient but the CI tests on travis-ci are configured for 0.12)

- This breaks compatibility with libjson0-dev,.... Needs libjson-c2 and/or libjson-c-dev!

- Tested only on travis-ci with the few unit tests.