Percy should not run on release branches

Created by: slimsag

As soon as we start cutting patches, Percy is broken because it is out of sync with master.

• https://github.com/sourcegraph/sourcegraph/commits/3.4
• https://github.com/sourcegraph/sourcegraph/commits/3.3
• https://github.com/sourcegraph/sourcegraph/commits/3.2

etc.