Fix the metric collection on postsubmit. (#2972)

* Fix the metric collection on postsubmit.

Turns out that the TRAVIS_PULL_REQUEST is not empty, but instead set to
"false".

scripts/collect_metrics.sh

@@ -33,7 +33,7 @@ EOF
   exit 1
 fi
 
-if [[ ! -z "${TRAVIS_PULL_REQUEST}" ]]; then
+if [[ "${TRAVIS_PULL_REQUEST}" != "false" ]]; then
   WORKSPACE="$1"
   SCHEME="$2"