Consistently add buildN candidate URL to gecko@ ports

Thunderbird 31.7.0 wasn't actually released yet despite
THUNDERBIRD_31_7_0_RELEASE tag in comm-esr31 hg repo 1 week old.
Based on releng IRC logs it seems there was an issue with automation.
Hopefully, upstream doesn't abandon 31.7.0 in favor of 38.0.

To avoid in future testing patience (BROKEN vs. PORTEPOCH) due to
discrepancy with release announcments let's fall back to candidates.

Reported by: pkg-fallout
MFH: 2015Q2
X-MFH-With: r386162