Fix #192 by updating Node JS version

[Botono]

The version of Node currently being used is outdated, resulting in the following AWS error when running Goad:

InvalidParameterValueException: The runtime parameter of nodejs8.10 is no longer supported for creating or updating AWS Lambda functions. We recommend you use the new runtime (nodejs12.x) while creating or updating functions.

This PR updates the Node version to 12.x. I built a binary from source on OSX and was successfully able to deploy a Node 12.x Lambda function and run a load test.

Regional results

Region: us-east-1
   TotReqs   TotBytes    AvgTime    AvgReq/s  (post)unzip
        20     9.1 kB     2.034s        4.61     2.1 kB/s
   Slowest    Fastest   Timeouts  TotErrors
    4.269s     0.128s          0          0

Overall

   TotReqs   TotBytes    AvgTime    AvgReq/s  (post)unzip
        20     9.1 kB     2.034s        4.61     2.1 kB/s
   Slowest    Fastest   Timeouts  TotErrors
    4.269s     0.128s          0          0
HTTPStatus   Requests
       200         20