Add support for nodejs8.10 (dherault#382)

Author: gcphost

src/index.js

@@ -315,7 +315,7 @@ class Offline {
     const apiKeys = this.service.provider.apiKeys;
     const protectedRoutes = [];
 
-    if (['nodejs', 'nodejs4.3', 'nodejs6.10', 'babel'].indexOf(serviceRuntime) === -1) {
+    if (['nodejs', 'nodejs4.3', 'nodejs6.10', 'nodejs8.10', 'babel'].indexOf(serviceRuntime) === -1) {
       this.printBlankLine();
       this.serverlessLog(`Warning: found unsupported runtime '${serviceRuntime}'`);