README: Improve links to coverage/CI to point to master.

Apparently, the CircleCI and Codecov links (and the Codecov badge)
weren't pointing specifically at master, so they'd sometimes show
state from the lastest push to a pull request, which isn't a
reasonable way to advertise whether the project's build is passing.

Author: timabbott

README.md

@@ -8,8 +8,8 @@ allows users to easily process hundreds or thousands of messages a day. With
 over 500 contributors merging over 500 commits a month, Zulip is also the
 largest and fastest growing open source group chat project.
 
-[![CircleCI branch](https://img.shields.io/circleci/project/github/zulip/zulip/master.svg)](https://circleci.com/gh/zulip/zulip)
-[![Coverage Status](https://img.shields.io/codecov/c/github/zulip/zulip.svg)](https://codecov.io/gh/zulip/zulip)
+[![CircleCI branch](https://img.shields.io/circleci/project/github/zulip/zulip/master.svg)](https://circleci.com/gh/zulip/zulip/tree/master)
+[![Coverage Status](https://img.shields.io/codecov/c/github/zulip/zulip/master.svg)](https://codecov.io/gh/zulip/zulip/branch/master)
 [![Mypy coverage](https://img.shields.io/badge/mypy-100%25-green.svg)][mypy-coverage]
 [![GitHub release](https://img.shields.io/github/release/zulip/zulip.svg)](https://github.com/zulip/zulip/releases/latest)
 [![docs](https://readthedocs.org/projects/zulip/badge/?version=latest)](https://zulip.readthedocs.io/en/latest/)

tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py

@@ -20,7 +20,7 @@
     'https://www.git-tower.com/blog/command-line-cheat-sheet/',
     'https://marketplace.visualstudio.com/items?itemName=rafaelmaiolla.remote-vscode',
     # Requires authentication
-    'https://circleci.com/gh/zulip/zulip',
+    'https://circleci.com/gh/zulip/zulip/tree/master',
     'https://circleci.com/gh/zulip/zulip/16617',
     'https://www.linkedin.com/company/zulip-project',
 ]