Add Travis acceptance tests, but don't make failures fatal yet /cc @arturi @hedgerh

.travis.yml

@@ -24,6 +24,8 @@ before_script:
 script:
   - npm run build
   - npm run test
+  # Allow failures until we figure out: StaleElementReferenceError: stale element reference: element is not attached to the page document
+  - npm run test:acceptance || true
   # Automatically deploy website onto gh-pages (if master and not PR)
   # Don't do this in `after_success`, because then failures won't bubble up
   - git config --global user.name 'Uppy Bot'