Rest-client atom package updates

I had a bit of time today to close some open issues:

• The oldest one was Fix Travis CI environment, at some point the way the test environment for Atom works changed and we were outdated, that’s now fixed. We sometimes have a problem with a file, but we’ll need more time to fix that, it seems a timeout problem or an unlink file one.

• We catched an exception to fix wrong json payload. If you add an invalid json payload now you’ll get a nice error.

• It’s possible to use a proxy server with the rest-client, wasn’t possible before.

For the future we’ll need to improve the test coverage and the UI for the Advanced options (like strict ssl, proxy server… it’s ugly now).