installation: fix prerequisite package versions
- FIX Adds upper limit on versions of several prerequisite packages (Babel, Flask-Login, Flask-RESTful, Flask-SQLAlchemy, lxml, Pillow, WTForms, etc) fixing Invenio 2.0 installation troubles.
- Amends the external collections getter test as per the new content of the inveniosoftware.org project site.
- Amends the documentation with respect to the code snippet highlights that were failing with certain Sphinx versions.
- Adds travis_retry to the Travis CI documentation building step that was sometimes failing due to the intersphinx inventory fetching troubles.
Reported-by: Dmitri Pisarev <dimaip@gmail.com>
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>