Don't insert stackmap frames into class files with version < 1.6. (#667)

For certain probes additional frames needs to be inserted, but only from
class file version 1.6 on.
diff --git a/org.jacoco.doc/docroot/doc/changes.html b/org.jacoco.doc/docroot/doc/changes.html
index ee53442..33fa7de 100644
--- a/org.jacoco.doc/docroot/doc/changes.html
+++ b/org.jacoco.doc/docroot/doc/changes.html
@@ -20,6 +20,13 @@
 
 <h2>Snapshot Build @qualified.bundle.version@ (@build.date@)</h2>
 
+<h3>Fixed Bugs</h3>
+<ul>
+  <li>Don't insert stackmap frames into class files with version &lt; 1.6,
+      this fixes regression which was introduced in version 0.6.5
+      (GitHub <a href="https://github.com/jacoco/jacoco/issues/667">#667</a>).</li>
+</ul>
+
 <h2>Release 0.8.1 (2018/03/21)</h2>
 
 <h3>New Features</h3>