Lower the requirement to Java 7

Now that we are using the error-prone javac, there is no need
to impose Java 8. However, because some of our tests are targetting
Java 8, developer should still use a JDK 8 to tests.

--
MOS_MIGRATED_REVID=96400914
diff --git a/src/BUILD b/src/BUILD
index 6a73011..e5957de 100644
--- a/src/BUILD
+++ b/src/BUILD
@@ -48,7 +48,7 @@
           VERSION_LINE=$$(cat $< | grep target_version);
           JAVA_VERSION=$$(echo $${VERSION_LINE} | sed -E 's/.*value="([^"])".*/\\1/');
           if [ -z "$${JAVA_VERSION}" ]; then
-            echo "1.8" >$@  # Java 8 is the default
+            echo "1.7" >$@  # Java 7 is the default
           elif [[ "$${JAVA_VERSION}" =~ ^[0-9]+$$ ]]; then
             echo "1.$${JAVA_VERSION}" >$@  # Add 1. before 7 or 8
           else