fix(socketio): allow emit to be array of strings in validate-script

[bernardobridge]

Why

When running this valid Socketio interface (since this PR: #1112):

flow:
   - emit:
       - "join"
       - "lobby"

You would get this error Scenario validation error: "scenarios[0].flow[0].emit" must be of type object.

Testing

Tested with a scenario locally. Automated tests for this will be added as part of another PR, as I'm adding tests for socketio in artillery package soon (only exist in core, which makes things like this fall through the cracks).