use pip cache for travis

Author: Someguy123

.travis.yml

@@ -2,6 +2,7 @@ dist: bionic
 services:
   - redis-server
 language: python
+cache: pip
 env:
   - CODECOV_TOKEN="e309d317-ca30-4c79-a01c-f4dc7c60e471"
 python:
@@ -15,8 +16,8 @@ before_install:
   - sudo apt-get update -qy
   - sudo apt-get install -qy iputils-ping
 install:
-  - pip install -r docs/requirements.txt
-  - pip install .
+  - pip install -U -r docs/requirements.txt
+  - pip install -U .
 script: pytest --cov=./privex -v
 after_success:
   - codecov