Bump version to 2.2.12

configure.ac

 dnl Process this file with autoconf to produce a configure script.
 
 dnl graph-tool package version number
-dnl An odd micro number indicates in-progress development.
-dnl An even micro number indicates a released version.
 m4_define(graph_tool_version_major, 2)
 m4_define(graph_tool_version_minor, 2)
-m4_define(graph_tool_version_micro, 12dev)
+m4_define(graph_tool_version_micro, 12)
 
 AC_INIT([graph-tool],[graph_tool_version_major().graph_tool_version_minor().graph_tool_version_micro()],[http://graph-tool.skewed.de])