Add labels to show how many builds have been triggered by PR

[John Vandenberg]

Useful to see who isnt testing locally.

[Eliran Pe'er]

That one is a bit tricky: TravisBuddy doesn't have the required permission to create labels

[John Vandenberg]

The labels could be created by the repo owners, and have to match a basic pattern that travisbuddy can detect.

e.g. a repo might have labels

• PR build count - 1
• PR build count - 2
• PR build count - 3
• PR build count - 4
• PR build count - 5
• PR build count - 10
• PR build count - 20
• PR build count - 30
• PR build count - 40
• PR build count - 50
• PR build count - >50