robot-name: Narrow constraints for robot names

The previous version of the regex that checks the robot names would also
accept names that didn't quite fit the pattern of the README. Wrong but
accepted robot names were:
- "12345"
- "AB123xyz"

This commit changes the regex to catch bad robot names like these.
Merge pull request #97 from sjakobi/fix-robot-name

robot-name: Narrow constraints for robot names