chore(travis): don't break the build when travis cache is empty

`du` returns error code 2 when any of the directories don't exist which breaks the build.

this scenario is common when the cache was emptied or when travis is building forks that don't have travis cache enabled

Author: IgorMinar

.travis.yml

@@ -38,7 +38,8 @@ matrix:
     - env: "JOB=e2e TEST_TARGET=jquery BROWSER_PROVIDER=browserstack"
 
 install:
-  - du -sh ./node_modules ./bower_components/ ./docs/bower_components/
+  # Check the size of caches
+  - du -sh ./node_modules ./bower_components/ ./docs/bower_components/ || true
   # - npm config set registry http://23.251.144.68
   # Disable the spinner, it looks bad on Travis
   - npm config set spin false