travis.yml: add manual submodules update

We have to update submodules from https, not ssh in travis.
For additional information see:
https://stackoverflow.com/questions/15674064/how-to-fix-a-permission-denied-publickey-error-for-a-git-submodule-update-in-t

Author: Snyssfx

.travis.yml

@@ -8,3 +8,11 @@ dist: trusty
 
 notifications:
    slack: se-spb:PszbuZP2YFs4mAY90xiTYldd
+
+# 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