Restrict def email notification to committer and author

Regardless of the branch, email only the committer and author of the
commit.
See
https://github.com/travis-ci/travis-ci/issues/488#issuecomment-29527429
and ensuing discussion on the rationale.
Merge pull request #297 from travis-ci/ha-feature-restrict_email_recipients

Restrict def email notification to committer and author