Drop back down to the default timeout for caching on CI

Building and uploading the cache now takes about 130 seconds on the
stable channel.  On beta and nightly it takes around 90 seconds.  The
cache on stable is larger because of additional clippy and npm files.

The caching behavior seems to be consistent enough to switch back to
the default setting of 180 seconds.

Author: jtgeibel

.travis.yml

@@ -11,8 +11,6 @@ cache:
   cargo: true
   directories:
     - $HOME/.npm
-  # Set timeout to 6 minutes (double the default of 3 minutes)
-  timeout: 360
 before_cache: script/ci/prune-cache.sh
 
 env: