Fix Snap build failure (#7)

- Updated build-package: of "gutenprint" part to include curl a the versioning automation scriptlet uses curl.
- Use GiT master instead of last release of pappl-retrofit, he current release of pappl-retrofit does not build 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

@@ -100,10 +100,11 @@ 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:
@@ -540,6 +541,7 @@ parts:
       - chrpath
       - libtool-bin
       - jq
+      - curl
     organize:
       snap/gutenprint-printer-app/current/usr/share: usr/share
       usr/lib/cups/filter/rastertogutenprint.5.3: usr/lib/gutenprint-printer-app/filter/rastertogutenprint.5.3