[ Robert Edmonds ]
* Release 1.3.1.

* Restore protobuf-2.x compatibility (#284, #285).

* Use xenial and protobuf 3.6.1 in the Travis-CI environment (#332).

* Convert uses of protobuf's scoped_ptr.h to C++11 std::unique_ptr, needed
  to compile against protobuf 3.6.1 (#320, #333).

* Use AX_CXX_COMPILE_STDCXX macro to enable C++11 support in old compilers
  (#312, #317, #327, #334).

[ Fredrik Gustafsson ]
* Add std:: to some types (#294, #305, #309).

[ Sam Collinson ]
* Check the return value of int_range_lookup before using as an array
  index; it can return -1 (#315).

[ Matthias Dittrich ]
* Fix compilation on mingw by using explicit protoc --plugin=NAME=PATH
  syntax in Makefile.am (#289, #290).