[gtkmm-documentation] Use latest gtkmm and glibmm versions.

[Murray Cumming <murrayc src gnome org>]

commit b158888c135f78aef6ed65a87919f265afa82b09
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Aug 25 16:20:44 2017 +0200

    Use latest gtkmm and glibmm versions.

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 70d8c1c..98a1d51 100644
--- a/configure.ac
+++ b/configure.ac
@@ -25,8 +25,8 @@ MM_AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory])
 AC_PATH_PROG([PERL], [perl], [perl])
 AC_PROG_SED
 
-PKG_CHECK_MODULES([GIOMM], [giomm-2.54 >= 2.51.1])
-PKG_CHECK_MODULES([GTKMM], [gtkmm-4.0  >= 3.89.0])
+PKG_CHECK_MODULES([GIOMM], [giomm-2.56 >= 2.55.1])
+PKG_CHECK_MODULES([GTKMM], [gtkmm-4.0  >= 3.91.2])
 MM_PKG_CONFIG_SUBST([GTHREAD_CFLAGS], [--cflags-only-other gthread-2.0])
 
 # glib-compile-resources and C compiler are needed to generate and compile