Fix Promise warning on write process

[deadratfink]

On write process while transforming YAML -> JS (yaml2js)

jyt -s swagger.yaml

I got this Warning message:

INFO: cannot resolve target type from file storing relative to input file, using default: js
INFO: origin: yaml
INFO: target: js
INFO: src:    swagger.yaml
INFO: dest:   swagger.js
INFO: indent: 4
INFO: Calling transformation: yaml2js
Warning: a promise was created in a handler but was not returned from it
    at writeToFile (/usr/local/lib/node_modules/jy-transform/lib/writer.js:183:12)
    at /usr/local/lib/node_modules/jy-transform/lib/writer.js:452:37
From previous event:
    at /usr/local/lib/node_modules/jy-transform/lib/writer.js:451:34
From previous event:
    at /usr/local/lib/node_modules/jy-transform/lib/writer.js:442:24
From previous event:
    at Writer.writeJs (/usr/local/lib/node_modules/jy-transform/lib/writer.js:441:14)
    at /usr/local/lib/node_modules/jy-transform/lib/transformer.js:94:24
    at processImmediate [as _immediateCallback] (timers.js:383:17)
From previous event:
    at itmo (/usr/local/lib/node_modules/jy-transform/lib/transformer.js:93:14)
    at Object.Transformer.yamlToJs [as yaml2js] (/usr/local/lib/node_modules/jy-transform/lib/transformer.js:131:16)
    at /usr/local/lib/node_modules/jy-transform/lib/transformer.js:421:56
From previous event:
    at Transformer.transform (/usr/local/lib/node_modules/jy-transform/lib/transformer.js:419:10)
    at Object.main (/usr/local/lib/node_modules/jy-transform/jyt:60:24)
    at after (/usr/local/lib/node_modules/jy-transform/node_modules/cli/cli.js:1057:18)
    at Object.cli.main (/usr/local/lib/node_modules/jy-transform/node_modules/cli/cli.js:1062:9)
    at Object.<anonymous> (/usr/local/lib/node_modules/jy-transform/jyt:80:5)
    at Module._compile (module.js:435:26)
    at Object.Module._extensions..js (module.js:442:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:313:12)
    at Function.Module.runMain (module.js:467:10)
    at startup (node.js:136:18)
    at node.js:963:3

Anyway, write process works properly