Start using Travis-CI cache (#6003)

.travis.yml

@@ -53,3 +53,15 @@ after_failure:
 
 after_success:
   - pwsh -File tools/travis.ps1 -Stage Success
+
+# travis-ci will quit using the cache if an enviroment variable changes
+env:
+  - CACHE_VERSION=netcoreapp.2.0.5
+
+# timeout uploading cache after 6 minutes (360 seconds)
+cache:
+  timeout: 360
+  directories:
+    - $HOME/.nuget
+    - $HOME/.dotnet
+    - $HOME/Library/Caches/Homebrew