dependencies: bump minimum requirements to fix installation with minimum versions

When manually setting all dependency requirements to the minimum version in
setup.py, as follows:

sed -i 's/>=/==/' setup.py dev_requirements.txt

and running:

pip install --upgrade -e . -r dev_requirements.txt python-ldap python-pam

following warnings were given:

sphinx 1.7.9 has requirement babel!=2.0,>=1.3, but you'll have babel 0.9.6 which is incompatible.
sphinx 1.7.9 has requirement docutils>=0.11, but you'll have docutils 0.8.1 which is incompatible.
sphinx 1.7.9 has requirement Pygments>=2.0, but you'll have pygments 1.5 which is incompatible.

Fix these by bumping the minimum versions of these dependencies.