optional readyFile suffix

Fixes https://github.com/bermi/sauce-connect-launcher/issues/39

Usage:  specifying `readyFileId: "123"` results in readyFile being created as `[temp-directory]/sc-launcher-readyfile_123`

In our CI environment we pass the build ID as the `readyFileId` so that multiple builds on the same worker box won't conflict with each other.