[build] Enable package cache for Linux builds.

Breakthrough · Breakthrough · commit 0f80f4cfd8f2 · 2022-05-29T00:04:40.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,5 @@
 language: python
+cache: pip
 
 matrix:
   include:
