Bump requirements

@piotr.gawron and @jacek.lebioda as predicted, I have issues even upgrading just to Django 2.1.

I copy here the message from the commit 9be3713e:

fixed issue with view logout

WIP: After the update, is failing to validate forms with boolean fields. If the boolean fields are not present, like postponed field on the test test_validation of web.tests.forms.test_StudySubjectEditForm.StudySubjectEditFormTests.

not sure how to proceed with the changes, apparently this should have raised issues before, because the docs indicate the following: https://docs.djangoproject.com/en/2.1/ref/forms/fields/#booleanfield

test_validation (web.tests.forms.test_StudySubjectEditForm.StudySubjectEditFormTests) ...

• postponed
  • This field is required.

Closes #350 (closed)