Modify submodule config on Travis before update

I guess this is a known issue when using Travis and SSH-based
submodules. The other option is to change the submodule config to be
HTTPS-based, but this should work.

.travis.yml

@@ -2,11 +2,19 @@ sudo: required
 language: python
 python: "3.6"
 services: docker
+git:
+    submodules: false
 env:
   global:
     - TESTER_IMAGE_NAME=atst-tester
     - PROD_IMAGE_NAME=atst-prod
 
+before_install:
+    # Use sed to replace the SSH URL with the public URL
+    - sed -i 's/git@github.com:/https:\/\/github.com\//' .gitmodules
+    # Manually initialize submodules
+    - git submodule update --init --recursive
+
 before_script:
   - docker login -u $ATAT_DOCKER_REGISTRY_USERNAME -p $ATAT_DOCKER_REGISTRY_PASSWORD $ATAT_DOCKER_REGISTRY_URL
   - docker build --tag "${TESTER_IMAGE_NAME}" . -f deploy/docker/tester/Dockerfile