Log Message:
jansson: updated to 2.12

Version 2.12

* Bug fixes:
  - Fix error message in `json_pack()` for NULL object.
  - Avoid invalid memory read in `json_pack()`.
  - Call va_end after va_copy in `json_vsprintf()`.
  - Improve handling of formats with '?' and '*' in `json_pack()`.
  - Remove inappropriate `jsonp_free()` which caused segmentation fault in
    error handling.

* Build:
  - Add function attributes for GCC and CLANG to provide warnings on improper
    use of jansson routines.
  - Many CMake fixes.
  - Enable -Bsymbolic-functions linker flag whenever possible.
  - Resolve various compiler warnings.
  - Fix code coverage ignored paths.

* Other:
  - Test coverage improvements.
  - Add VS 2017 to appveyor, update Visual Studio documentation.
  - Update copyright for 2018.
  - Update install instructions in README