Use git for the snap version (#6271)

For commits with an annotated tag, that tag will be the version. For other commits, a git identifier will be used.
This way, there is no need to manually edit the snap version.

snap/snapcraft.yaml

@@ -1,5 +1,5 @@
 name: parity
-version: master
+version: git
 summary: Fast, light, robust Ethereum implementation
 description: |
   Parity's goal is to be the fastest, lightest, and most secure Ethereum