Lintly fails in CodeBuild when merged to master

[grantmcconnaughey]

When running CodeBuild on merges Lintly will fail. It only takes the format of pr/{pr_number} if triggered by a webhook.

https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-env-vars.html

Traceback (most recent call last): 
  File "/usr/local/bin/lintly", line 10, in <module> 
    sys.exit(main()) 
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 722, in __call__ 
    return self.main(*args, **kwargs) 
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 697, in main 
    rv = self.invoke(ctx) 
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 895, in invoke 
    return ctx.invoke(self.callback, **ctx.params) 
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 535, in invoke 
    return callback(*args, **kwargs) 
  File "/usr/local/lib/python3.6/site-packages/lintly/cli.py", line 67, in main 
    build.execute() 
  File "/usr/local/lib/python3.6/site-packages/lintly/builds.py", line 55, in execute 
    if not self.config.pr: 
  File "/usr/local/lib/python3.6/site-packages/lintly/config.py", line 25, in pr 
    return self.cli_config['pr'] or getattr(self.ci, 'pr') 
  File "/usr/local/lib/python3.6/site-packages/lintly/ci.py", line 103, in pr 
    return os.environ['CODEBUILD_SOURCE_VERSION'].split('/')[1] 
IndexError: list index out of range