Use snapcraft docker image from beta tag as it builds more often (#342)

.travis.yml

@@ -5,7 +5,7 @@ sudo: required
 services: [docker]
 
 script:
-  - docker run -v $(pwd):$(pwd) -w $(pwd) snapcore/snapcraft:stable sh -c "apt update && snapcraft"
+  - docker run -v $(pwd):$(pwd) -w $(pwd) snapcore/snapcraft:beta sh -c "apt update && snapcraft"
   - sudo apt update
   - sudo apt install --yes snapd
   - sudo snap install *.snap --classic --dangerous