Fix CI (#98)

## What is the goal of this PR?

As it turns out, we still need to use Python 2 in `deploy-github`, just at a very specific stage of it.

## What are the changes implemented in this PR?

Before executing the actual deployment to GitHub, we switch back to Python 2 and clean the Bazel sandbox such that PyPI dependencies are refetched

Author: vmax

.circleci/config.yml

@@ -136,6 +136,8 @@ jobs:
           pip install certifi
           export RELEASE_NOTES_TOKEN=$REPO_GITHUB_TOKEN
           bazel run @graknlabs_build_tools//ci:release-notes -- client-python $(cat VERSION) ./RELEASE_TEMPLATE.md
+      - run: pyenv global 2.7.12 3.5.2
+      - run: bazel clean --expunge
       - run: |
           export DEPLOY_GITHUB_TOKEN=$REPO_GITHUB_TOKEN
           bazel run --define version=$(cat VERSION) //:deploy-github -- $CIRCLE_SHA1