Add a timeout to upload calls

[sslotsky]

We are currently encountering an issue in Travis where some upload attempts are hanging.

https://www.traviscistatus.com/incidents/kyf149kl6bvp

The Travis build will abort after 10 minutes of not receiving output. Adding a timeout to the request.post call could be very helpful here. When I forked this library and added such a timeout, the plugin failed the build with this error:

ERROR in RollbarSourceMapPlugin: failed to upload 23.0a01a4daa723d7b93f81.js.map to Rollbar: ESOCKETTIMEDOUT

Having the plugin fail the build IMO is much better than having Travis wait 10 minutes and then abort. Not only do we get faster and more accurate feedback, but we can also use the ignoreErrors option and accept a partial success, allowing us to deploy without disabling the plugin completely.