fixing npm@5 and npm@6 by only running npm prune on npm@4

Author: mansona

lib/dependency-manager-adapters/npm.js

@@ -104,7 +104,15 @@ module.exports = CoreObject.extend({
 
     return this.run(cmd, [].concat(['install'], mgrOptions), { cwd: this.cwd }).then(() => {
       if (!adapter.useYarnCommand) {
-        return adapter.run(adapter.configKey, ['prune'], { cwd: adapter.cwd });
+        return adapter.run('npm', ['--version'], { cwd: this.cwd, stdio: 'pipe' }).then((res) => {
+          let version = res.stdout;
+          if (version.match(/^4./)) {
+            debug('Running npm prune because version is %s', version);
+            return adapter.run(adapter.configKey, ['prune'], { cwd: adapter.cwd });
+          }
+
+          debug('Not running npm prune because version is %s', version);
+        })
       }
     });
   },