Change SSH to GIT URL for submodule.

Got an error from Travis CI:

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$ git submodule update

Cloning into 'js/jquery/jquery-handsontable-src'...

Warning: Permanently added the RSA host key for IP address '192.30.252.131' to the list of known hosts.

Permission denied (publickey).

fatal: Could not read from remote repository.

Please make sure you have the correct access rights and the repository exists.

Clone of 'git@github.com:collectiveaccess/jquery-handsontable.git' into submodule path 'js/jquery/jquery-handsontable-src' failed

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This will (hopefully) fix that.