build: bump libbson requirement to 0.4.0.

Author: Christian Hergert

configure.ac

@@ -158,7 +158,7 @@ AM_CONDITIONAL([HAVE_PYTHON], [test "x$ax_python_header" != "xno"])
 dnl **************************************************************************
 dnl Check for libbson
 dnl **************************************************************************
-PKG_CHECK_MODULES(BSON, libbson-1.0 >= 0.2.3)
+PKG_CHECK_MODULES(BSON, libbson-1.0 >= 0.4.0)
 
 
 dnl **************************************************************************