Add async to migration skeleton functions

[townofdon]

This is a minor enhancement and would speed up workflow for those using async/await.

All tests passing, except for the following which seems unrelated to my changes:

290 passing (9m)
  1 failing

  1) [SQLITE] lib/sequelize model:create
       attributes
         --attributes first_name:string,last_name:string,bio:text,role:enum:{Admin,"Guest User"},reviews:array:text
           exits with exit code 0:
     Uncaught Error: Command failed: /Users/don/websites/js/sequelize/cli/lib/sequelize model:create --name User --attributes first_name:string,last_name:string,bio:text,role:enum:{Admin,"Guest User"},reviews:array:text
ERROR: Attribute 'role:enum:Admin' cannot be parsed: Unknown type 'Admin'

      at ChildProcess.exithandler (child_process.js:288:12)
      at ChildProcess.EventEmitter.emit (domain.js:442:20)
      at maybeClose (internal/child_process.js:962:16)
      at Process.ChildProcess._handle.onexit (internal/child_process.js:249:5)

[SimonSchick]

Not every code base uses node v8 yet and this would be extra churn, not merging before we go v8+

[eseliger]

what else would work is adding a process.version check, if we want to proceed with this more quickly