Install multipass for snapcraft

You need multipass installed to build snaps (https://github.com/CanonicalLtd/multipass).
Would you like to install it now? [y/N]:

Author: techtonik

.travis.yml

@@ -25,7 +25,9 @@ jobs:
   - stage: Pack snap
     env: EMPTY
     before_install: skip
-    install: sudo snap install snapcraft --classic
+    install:
+    - sudo snap install snapcraft --classic
+    - sudo snap install multipass --classic --beta
     script: snapcraft
     deploy:
       on: