atom-shell v0.20.8

Changelog:

• Node symbols are no longer inserted in iframes.
• Fix a race condition which caused node integration not correctly initialized.
• Add https://atom.io/download/atom-shell/index.json web endpoint which
  contains version information of all atom-shell releases.
• Fix MediaStreamTrack.getSources returning sources with empty labels.
• Fix receiving IPC messages in <webview> when node integration is off.
• Add Plus as accelerator key.