Gracefully handle github rate limiting in version check

[sbliven]

We request the latest version from github without authorization. This can run into rate limiting:

2. GitHub API Rate Limiting
   GitHub limits unauthenticated API requests to 60 per hour per IP address.
   If you (or others on your shared office IP) have made many requests recently, GitHub may be throttling you.

This should be handled better in the cli. It should not block using the cli. Long term, maybe we should mirror the version info elsewhere.

[sbliven]

Mitigated by #189 and #190, which add timeouts for the github API calls.