Increase -Xmx flag for junit runner.

The build.sh script was failing on OOM during running
tests (ddply test was the last test in the log).
Hence I increased heap value.

build.sh

@@ -193,7 +193,7 @@ function build_javadoc() {
 
 function junit() {
     echo "running JUnit tests..."
-    "$JAVA" -ea -cp ${JAR_FILE} water.Boot -mainClass water.JUnitRunner
+    "$JAVA" -Xmx2g -ea -cp ${JAR_FILE} water.Boot -mainClass water.JUnitRunner
 }
 
 if [ "$1" = "onlydoc" ]; then