Jansson 2.13 released

[Petri Lehtinen]

Jansson 2.13 has been released.

Changes:

* New Features:

- Add `jansson_version_str()` and `jansson_version_cmp()` for runtime
version checking (#465).

- Add `json_object_update_new()`, `json_object_update_existing_new()`
and `json_object_update_missing_new()` functions (#499).

- Add `json_object_update_recursive()` (#505).

* Build:

- Add ``-Wno-format-truncation`` to suppress format truncation warnings (#489).

* Bug fixes:

- Remove ``strtod`` macro definition for MinGW (#498).

- Add infinite loop check in `json_deep_copy()` (#490).

- Add ``pipe`` macro definition for MinGW (#500).

- Enhance ``JANSSON_ATTRS`` macro to support earlier C standard(C89) (#501).

- Update version detection for sphinx-build (#502).

* Documentation:

- Fix typos (#483, #494).

- Document that call the custom free function to free the return value
of `json_dumps()` if you have a custom malloc/free (#490).

- Add vcpkg installation instructions (#496).

- Document that non-blocking file descriptor is not supported on
`json_loadfd()` (#503).

Download: http://www.digip.org/jansson/releases/jansson-2.13.tar.bz2
Docs: http://jansson.readthedocs.io/en/2.13/