Solved public key bug.

Used https://stackoverflow.com/questions/15674064/how-to-fix-a-permission-denied-publickey-error-for-a-git-submodule-update-in-t/24600210

.travis.yml

@@ -1,4 +1,11 @@
 language: php
+# Handle git submodules yourself
+git:
+    submodules: false
+# Use sed to replace the SSH URL with the public URL, then initialize submodules
+before_install:
+    - sed -i 's/git@github.com:/https:\/\/github.com\//' .gitmodules
+    - git submodule update --init --recursive
 before_script:
     - travis_retry composer self-update
     - travis_retry composer install --no-interaction --prefer-source --dev