🐛 Don't load 'repl' unless needed

Git bisect revealed that process.stdout failed to initiate (and was replaced with a write stream that does nothing) when the 'repl' module was loaded. This commit moves `require('repl')` so that it is only loaded when the --interactive flag is present.

Fixes #5051

spec/node-spec.js

@@ -225,7 +225,7 @@ describe('node feature', function () {
       process.stdout.write('test')
     })
 
-    xit('should have isTTY defined', function () {
+    it('should have isTTY defined', function () {
       assert.equal(typeof process.stdout.isTTY, 'boolean')
     })
   })