Revert pip cache changes. Use pip's built-in retry.

tools/travis_notes.txt

@@ -14,8 +14,9 @@ the yaml file is valid.
      set -ex
    ```
 
-- Use the `retry` bash function from `before_install.sh` before a command to have it try 3 times before
-failing (useful for installing from third party sources).
+- Use the `retry` bash function from `before_install.sh` before a command to
+have it try 3 times before failing.
+- Use `pip install --retries N` for retrying package downloads.
 - Use the `section` function to start a folded section of the script.
 Section names must have underscores or dots instead of spaces and must be
 accompanied by a corresponding `section_end` call.