Updated Travis to build an image for testing

Previously we relied on the firefox and xvfb packages living in our images. This is no longer the case as these packages are not needed for production-like environments. We now create a new image, with the "test" tag, that includes these packages.

LEARNER-818

.travis/Dockerfile

+FROM edxops/ecommerce:latest
+
+RUN apt-get update && \
+    apt-get install --no-install-recommends -y \
+        firefox \
+        xvfb \
+    && rm -rf /var/lib/apt/lists/*

.travis/docker-compose-travis.yml

@@ -13,8 +13,9 @@ services:
       MYSQL_PASSWORD: "password"
       MYSQL_DATABASE: "ecommerce"
   ecommerce:
-    image: edxops/ecommerce:latest
+    image: edxops/ecommerce:test
     container_name: ecommerce_testing
+    build: .
     volumes:
       - ..:/edx/app/ecommerce/ecommerce
       - $HOME/.cache/pip:/edx/app/ecommerce/.cache/pip