Fix visual tests

.gemini-ff.yml

@@ -0,0 +1,16 @@
+rootUrl: http://localhost:8080/components/vaadin-overlay/test/visual/
+gridUrl: http://localhost:4444/wd/hub
+screenshotsDir: ./test/visual/screens
+
+system:
+  plugins:
+    polyserve:
+      port: 8080
+    sauce: true
+
+browsers:
+  firefox:
+    desiredCapabilities:
+      browserName: "firefox"
+      version: "47.0"
+      platform: "Windows 10"

.gemini.yml

@@ -14,9 +14,3 @@ browsers:
       browserName: "chrome"
       version: "67.0"
       platform: "Windows 10"
-
-  firefox:
-    desiredCapabilities:
-      browserName: "firefox"
-      version: "47.0"
-      platform: "Windows 10"

.travis.yml

@@ -40,7 +40,7 @@ script:
       if [[ "$TRAVIS_EVENT_TYPE" != "pull_request" && "$TRAVIS_BRANCH" != quick/* ]]; then
         if [[ "$TEST_SUITE" = "visual_tests" ]]; then
           npm i -q --no-save gemini@^4.0.0 gemini-sauce gemini-polyserve &&
-          gemini test test/visual;
+          gemini test test/visual && gemini test test/visual -c .gemini-ff.yml;
         else
           wct --env saucelabs:$TEST_SUITE;
         fi;