[CS2] Don’t require async/await support to run coffee (#4679)

* Get `coffee` command working again in Node 6, by converting the ‘await’ wrapper in the REPL to use a Promise instead of the ‘await’ keyword; add tests for REPL ‘await’ wrapper, including test to skip async tests if the runtime doesn’t support them

* Code review

* Let's support Node 6+ if we can

src/repl.coffee

@@ -44,9 +44,9 @@ replDefaults =
       result = runInContext js, context, filename
       # Await an async result, if necessary
       if isAsync
-        result = await result
-        cb null, result unless sawSIGINT
-        sawSIGINT = false
+        result.then (resolvedResult) ->
+          cb null, resolvedResult unless sawSIGINT
+        sawSIGINT = no
       else
         cb null, result
     catch err