Fallback on using service provider runtime / 'nodejs'

When using "sls deploy function --function=function-name",
serverless-webpack would crash if the runtime was not set explicitly for
the function. This patch makes behavior consistent with other accesses
to the function runtime, falling back on the service provider's runtime
(or 'nodejs') if the function doesn't declare a runtime.

Author: mostthingsweb

index.js

@@ -106,7 +106,7 @@ class ServerlessWebpack {
       'after:package:createDeploymentArtifacts': () => BbPromise.bind(this).then(this.cleanup),
 
       'before:deploy:function:packageFunction': () => {
-        const runtime = this.serverless.service.getFunction(this.options.function).runtime;
+        const runtime = this.serverless.service.getFunction(this.options.function).runtime || this.serverless.service.provider.runtime || 'nodejs';
 
         if (isNodeRuntime(runtime)) {
           return BbPromise.bind(this)