Fix RAMCloud JNI build script

- Do not depend on a symbolic link created by
  default-jdk package which may point to
  JDK version other than 7, depending on Ubuntu version.

Change-Id: Ifdde51d27c578038488b17bdf0a89584a5f303fb
diff --git a/ramcloud-build-scripts/build_jni_so.sh b/ramcloud-build-scripts/build_jni_so.sh
index 5e202fb..9145bb5 100755
--- a/ramcloud-build-scripts/build_jni_so.sh
+++ b/ramcloud-build-scripts/build_jni_so.sh
@@ -6,7 +6,7 @@
   if [ -z "${OPENJDK}" -a -d "/usr/lib/jvm/java-7-oracle" ]; then
     JAVA_HOME=/usr/lib/jvm/java-7-oracle
   else
-    JAVA_HOME=/usr/lib/jvm/default-java
+    JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64
   fi
   echo "Assuming JAVA_HOME=${JAVA_HOME}"
 fi