Support snap building and testing in Travis CI

This ensure snap build-time and run-time problems will be noticed more
promptly by the maintainers, like issue mikf#229.

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

.travis.yml

@@ -10,14 +10,25 @@ matrix:
   include:
     - python: "3.7"
       dist: xenial
+      env: SNAP_TESTS=false
     - python: "3.8-dev"
       dist: xenial
+      env: SNAP_TESTS=false
     - python: "3.6"
       env: GALLERYDL_TESTS=results
+      env: SNAP_TESTS=false
+    - dist: xenial
+      env: SNAP_TESTS=true
 
 git:
   depth: 3
   quiet: true
+
+addons:
+  snaps:
+    - name: snapcraft
+      classic: true
+
 branches:
   only:
   - master
@@ -26,3 +37,4 @@ branches:
 
 script:
   - ./scripts/run_tests.sh
+  - '[[ "${SNAP_TESTS}" = true ]] && snapcraft --destructive-mode && sudo snap try && snap run gallery-dl'