pipfile: pass --clear flag, and do it separetely to not be ignored

Maybe --clear flag works as part of `pipenv install` now, but code
inspection indicates to me it still don't. This resolves the issue with
the pipenv versions we currently use in tests at least.

repo2docker/buildpacks/pipfile/__init__.py

@@ -154,7 +154,8 @@ class PipfileBuildPack(CondaBuildPack):
                 "${NB_USER}",
                 """(cd {working_directory} && \\
                     PATH="${{KERNEL_PYTHON_PREFIX}}/bin:$PATH" \\
-                        pipenv install {install_option} --system --dev \\
+                        pipenv install {install_option} --system --dev && \\
+                        pipenv --clear \\
                 )""".format(
                     working_directory=working_directory,
                     install_option="--ignore-pipfile"