caching artifacts dir in CI

Author: johnnychen94

.travis.yml

@@ -10,6 +10,9 @@ julia:
   - nightly
 notifications:
   email: false
+cache:
+  directories:
+    - $HOME/.julia/artifacts
 # uncomment the following lines to override the default test script
 #script:
 #  - if [[ -a .git/shallow ]]; then git fetch --unshallow; fi