Enable TravisCI caching for pip and pre-commit plugins

This should speed up TravisCI builds.

.travis.yml

@@ -2,8 +2,15 @@
 dist: xenial
 language: python
 python: 3.7
+# pre-commit hooks can use Docker, so we should go ahead and enable it
 services: docker
 
+# Cache pip packages and pre-commit plugins to speed up builds
+cache:
+  pip: true
+  directories:
+    - $HOME/.cache/pre-commit
+
 install:
   - pip install --upgrade -r requirements-test.txt
 script: