Small cleanup to failure reporting script (#1339)

No need to construct the build URL – travis gives it to us!
diff --git a/tool/report_failure.sh b/tool/report_failure.sh
index 39d87a9..3a0d925 100755
--- a/tool/report_failure.sh
+++ b/tool/report_failure.sh
@@ -1,6 +1,5 @@
-TRAVIS_BUILDS_URI="https://travis-ci.org/dart-lang/test/builds"
 if [ "$TRAVIS_EVENT_TYPE" != "pull_request" -a "$TRAVIS_ALLOW_FAILURE" != "true" ]; then
   curl -H "Content-Type: application/json" -X POST -d \
-    "{'text':'Build failed! ${TRAVIS_BUILDS_URI}/${TRAVIS_BUILD_ID}'}" \
+    "{'text':'Build failed! ${TRAVIS_BUILD_WEB_URL}'}" \
     "${CHAT_HOOK_URI}"
 fi