Only build PROJECT=reference from kokoro/build.sh

We should not assume that all subdirectories of project/ are valid
project names.

Change-Id: I13db4344ccb3b0338fd1500572a1141f669796ed
diff --git a/kokoro/build.sh b/kokoro/build.sh
index db81d6e..af59b9e 100755
--- a/kokoro/build.sh
+++ b/kokoro/build.sh
@@ -76,10 +76,7 @@
 # Step 1: make sure it builds.
 #
 
-for proj in $(cd project/ && ls)
-do
-  make PROJECT=${proj}
-done
+make PROJECT=reference
 
 #
 # Step 2: make sure it works.