This change only slightly modifies the bahaviour of travis/runner.sh, but:
1. speeds up the tests, because it does not install dependencies before running
them. Dependencies are already installed by .travis.yml in its "install"
section;
2. if for some reason Etherpad does not start, there is a sudden failure,
instead of launching the front end tests anyway, and then having to wait 10
minutes for them to time out;
3. it is compatible with a different way of installing etherpad dependencies
("npm ci" instead of "npm install"), whereas the previous one broke. This
will probably be introduced in a while, so this change future-proofs for it
(see #3778).
4. it is more robust, because it detects more reliably the paths, and changes
between them correctly;
Please note that the script now requires bash instead of a generic posix shell.
This may break on platforms which default to a different shell (FreeBSD, MacOS?)
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