Merge pull request #88 from heroku/multi-friendly-caching

Blow away the cached node_modules directory instead of the entire cache.

bin/compile

@@ -72,7 +72,7 @@ status "Installing dependencies"
 npm install --userconfig $build_dir/.npmrc --production 2>&1 | indent
 
 status "Caching node_modules directory for future builds"
-rm -rf $cache_dir
+rm -rf $cache_dir/node_modules
 mkdir -p $cache_dir
 test -d $build_dir/node_modules && cp -r $build_dir/node_modules $cache_dir/