installation: relax version number constraints

• Relaxes all the hard constraints on version numbers.

• Comments that Cerberus 0.7.1 will fail.

• Comments Flask-OAuth>=0.5 to be known to fail.

• Comments pyparsing 2.0.2 to be known to fail.

• Comments SQLAlchemy-Utils >0.24 to be depending on SQLAlchemy>=0.9.3.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>