Update prompts for IOPIPE_TOKEN #74
Description
The docs now say to use environment variable, but the plugin still warns here:
Serverless: serverless-plugin-iopipe: You did not specify iopipeToken in your custom section of serverless.yml. iopipe will fall back to $IOPIPE_TOKEN in the lambda environment
about not setting up in the serverless.yml. This can be confusing to users.