Bump version number.

NEWS

+Changes in version 3.19.4 are:
+ * Fix up startup and initialization [#756324]
+ * Fix DBus "Type" property of org.freedesktop.Secret.Item [#759399]
+ * Build fixes [#753698]
+ * Updated translations
+
 Changes in version 3.18.3 are:
  * Fix regression looking up secrets in default keyring [#756865]
  * Updated translations

configure.ac

 AC_INIT(gnome-keyring,
-        3.18.3,
+        3.19.4,
         [gnome-keyring-list@gnome.org],
         [gnome-keyring],
         [https://wiki.gnome.org/Projects/GnomeKeyring])