Fix a reference leak in the number converter (#1078)

Fixes #1075.

`PyNumber_Float()` and `PyNumber_Long()` return new references.
diff --git a/docs/changelog.rst b/docs/changelog.rst
index 612a0a6..eea15d5 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -17,6 +17,9 @@
 * Fixed compilation with Clang on host GCC < 5 (old libstdc++ which isn't fully
   C++11 compliant). `#1062 <https://github.com/pybind/pybind11/pull/1062>`_.
 
+* Fixed a reference leak in the number converter.
+  `#1078 <https://github.com/pybind/pybind11/pull/1078>`_.
+
 * Fixed a regression where the automatic ``std::vector<bool>`` caster would
   fail to compile. The same fix also applies to any container which returns
   element proxies instead of references.