Support triggering from GitHub event "pull_request"

Upgrading to version 1.2 requires to add the trigger on pull_request in order to analyze pull requests, for example:

on:
  # Trigger analysis when pushing in master or pull requests, and when creating
  # a pull request. 
  push:
    branches:
      - master
  pull_request:
      types: [opened, synchronize, reopened]