Use GiT master instead of last realease of pappl-retrofit (#27)

The current release of pappl-retrofit does notbuild with current libcups2 (of CUPS 2.4.7). Therefore we need to include the fixes which got applied after the last release.

snap/snapcraft.yaml

@@ -105,10 +105,11 @@ parts:
   pappl-retrofit:
     source: https://github.com/openprinting/pappl-retrofit
     source-type: git
-    source-tag: '1.0b2'
+    # source-tag: '1.0b2'
     source-depth: 1
 # ext:updatesnap
 #   version-format:
+#     ignore: true
 #     format: '%V'
     plugin: autotools
     autotools-configure-parameters: