chore(ci): fix firefox tests on Travis (#4828)

I forgot Firefox requires explicit installation.

.travis.yml

@@ -15,10 +15,10 @@ before_install:
   - "sysctl kernel.unprivileged_userns_clone=1"
   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
-  - 'if [ "$NODE8" = "true" ]; then cd experimental/puppeteer-firefox && npm i && cd ../..; fi'
 script:
   - 'if [ "$NODE8" = "true" ]; then npm run lint; fi'
   - 'if [ "$NODE8" = "true" ]; then npm run coverage; fi'
+  - 'if [ "$FIREFOX" = "true" ]; then cd experimental/puppeteer-firefox && npm i && cd ../..; fi'
   - 'if [ "$FIREFOX" = "true" ]; then npm run funit; fi'
   - 'if [ "$NODE8" = "true" ]; then npm run test-doclint; fi'
   - 'if [ "$NODE8" = "true" ]; then npm run test-types; fi'