build: Add caching for pip files (open-telemetry#520)

Caches lets Travis CI store directories between builds, which is useful for storing dependencies that take longer to compile or download. -Travis CI Docs

Signed-off-by: Daniel González Lopes danielgonzalezlopes@gmail.com

Author: dgzlopes

.travis.yml

@@ -2,6 +2,8 @@ dist: xenial
 
 language: python
 
+cache: pip
+
 python:
   - '3.4'
   - '3.5'