travis: Simplify installation process.

.travis.yml

@@ -10,16 +10,9 @@ matrix:
   - python: "3.6"
     env: TEST_SUITE=lint
 install:
-    # Install codecov, the library for the code coverage reporting tool we use
   - pip install codecov
-
-    # Install pycodestyle, the library for checking pep8 code conformity.
-  - pip install pycodestyle==2.3.1
-
-    # Install all API packages
-  - pip install ./zulip
-  - pip install ./zulip_bots
-  - pip install ./zulip_botserver
+  - tools/provision
+  - source zulip-api-py*-venv/bin/activate
 script:
   - tools/$TEST_SUITE
 after_success: