Merge pull request #574 from mrakitin/workaround_pip-19.0.1_issues

FIX: a temporary workaround for pip-19.0.1 issues

Author: betatim

repo2docker/buildpacks/python/__init__.py

@@ -63,6 +63,7 @@ def get_assemble_scripts(self):
         if os.path.exists(requirements_file):
             assemble_scripts.append((
                 '${NB_USER}',
+                'pip install "pip<19" && ' + \
                 '{} install --no-cache-dir -r "{}"'.format(pip, requirements_file)
             ))