Travis: only use Tor tags that include "tbb"

This is to avoid accepting tags like "test_bug_30480-t4".

.travis/check-tbb-tags

@@ -9,7 +9,7 @@ GIT_TAG=$(git submodule status tor-browser-build | awk '{print $3}')
 
 GIT_URL=https://git.torproject.org/builders/tor-browser-build.git
 
-LATEST_TAG=$(git ls-remote --tags "${GIT_URL}" | grep -v '\^{}' | awk '{print $2}' | awk -F"/" '{print $3}' | sort -V | tail --lines=1)
+LATEST_TAG=$(git ls-remote --tags "${GIT_URL}" | grep 'tbb' | grep -v '\^{}' | awk '{print $2}' | awk -F"/" '{print $3}' | sort -V | tail --lines=1)
 if [ "${GIT_TAG}" != "(${LATEST_TAG})" ]
 then
     FAIL=1