Watching file changes not working with `serverless run`

[kbrandwijk]

I created a PR to support serverless run (#49), but watching for file changes is not working, and I don't know why.

[kbrandwijk]

serverless-webpack seems to have found a solution for that (serverless-heaven/serverless-webpack#270). After I have tested that, I will also apply the same solution here.

[dhessler]

@kbrandwijk would this also work for watching for file changes when using serverless-offline?