circleci: Downgrade to 2.0.

We initially upgraded to 2.1 under the assumption that 2.1 was necessary
to enable pipelines. This turns out to be not true. Pipelines can be enabled
from 2.0 as well. On the other hand upgrading to 2.1 requires enabling
pipelines. This resulted in broken builds across many forks that has enabled
CircleCI and not pipelines. We are also not making use of any 2.1 features
like reusable orbs, commands, and executors currently. Downgrading to 2.0 will
also allow us to use "." in the job names which makes them more readable.

.circleci/config.yml

@@ -3,7 +3,7 @@
 # See CircleCI upstream's docs on this config format:
 #   https://circleci.com/docs/2.0/language-python/
 #
-version: 2.1
+version: 2.0
 aliases:
   - &create_cache_directories
     run:
@@ -82,7 +82,7 @@ aliases:
          || echo "Error in uploading coverage reports to codecov.io."
 
 jobs:
-  "trusty-backend-frontend-py34":
+  "trusty-backend-frontend-python3.4":
     docker:
       # This is built from tools/circleci/images/trusty/Dockerfile .
       # Trusty ships with Python 3.4.
@@ -108,7 +108,7 @@ jobs:
       #     # also /tmp/zulip-test-event-log/
       #     destination: test-reports
 
-  "xenial-backend-py35":
+  "xenial-backend-python3.5":
     docker:
       # This is built from tools/circleci/images/xenial/Dockerfile .
       # Xenial ships with Python 3.5.
@@ -127,7 +127,7 @@ jobs:
       - *run_backend_tests
       - *upload_coverage_report
 
-  "bionic-backend-py36":
+  "bionic-backend-python3.6":
     docker:
       # This is built from tools/circleci/images/bionic/Dockerfile .
       # Bionic ships with Python 3.6.
@@ -159,6 +159,6 @@ workflows:
   version: 2
   build:
     jobs:
-      - "trusty-backend-frontend-py34"
-      - "xenial-backend-py35"
-      - "bionic-backend-py36"
+      - "trusty-backend-frontend-python3.4"
+      - "xenial-backend-python3.5"
+      - "bionic-backend-python3.6"