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.

setup.py

@@ -47,13 +47,13 b' requirements = ['
     "FormEncode >= 1.2.4, < 1.4",
     "SQLAlchemy >= 1.1, < 1.3",
     "Mako >= 0.9.0, < 1.1",
-    "Pygments >= 1.5, < 2.3",
+    "Pygments >= 2.0, < 2.3",
     "Whoosh >= 2.5.0, < 2.8",
     "celery >= 3.1, < 4.0", # celery 4 doesn't work
-    "Babel >= 0.9.6, < 2.7",
+    "Babel >= 1.3, < 2.7",
     "python-dateutil >= 1.5.0, < 2.8",
     "Markdown >= 2.2.1, < 2.7",
-    "docutils >= 0.8.1, < 0.15",
+    "docutils >= 0.11, < 0.15",
     "URLObject >= 2.3.4, < 2.5",
     "Routes >= 1.13, < 2",
     "dulwich >= 0.14.1, < 0.20",