Assigner 1.2.0

This version primarily provides some performance improvements that make operating on large rosters much faster -- we're seeing about a 50% speedup in our testing. These improvements should also reduce TLS handshake errors.

We've also added a few UI tweaks: assigner status prints each student's branch names on their own line and there's now a --version flag!

Changes from 1.1.1:

• Branch names are shown each on their own line in assigner status to make output more legible
• Send GitLab authentication token in HTTP header rather than request parameters
• Print informative message if git is not installed
• Use a shared HTTPS session for API calls to speed up operations
• Add --version flag to Assigner