Enable Welcome Message to post review comments

After extensive fixing from Jeremey and the Infrastructure
team, welcome message now detects newbies correctly.

The code has been run for a week in dry-run mode, and has
been verified to produce no false positives.

This patch removes the dry run option from the hook, to enable
welcome message to post welcoming messages on reviews from first-
time contributors.

Change-Id: I5dd072d36480294eefc23aa6376ae0a686621b08
Merge "Enable Welcome Message to post review comments"