release: bump version to 1.20.20

Author: dkosovic

configure.ac

@@ -1,7 +1,7 @@
 AC_PREREQ([2.69])
 
 AC_INIT([NetworkManager-l2tp],
-        [1.20.18],
+        [1.20.20],
         [https://github.com/nm-l2tp/NetworkManager-l2tp/issues],
         [NetworkManager-l2tp])
 AM_INIT_AUTOMAKE([1.9 foreign no-dist-gzip dist-xz -Wno-portability])