Travis CI: Include npm prune in the before_script command.

Caching of node modules was added in [37058]. `npm prune` removes extraneous packages so the cache contains only current modules.

Props netweb.
Fixes #36490.

git-svn-id: https://develop.svn.wordpress.org/trunk@37340 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -50,6 +50,7 @@ before_script:
 - npm install -g npm
 - npm install -g grunt-cli
 - npm install
+- npm prune
 script: grunt $WP_TRAVISCI
 notifications:
   slack: