Switch commoncpp2 to ucommon to solve dependency conflicts.

libccrtp was depending on commoncpp2, and have been replaced by a version
depending on ucommon as well.
diff --git a/jni/libucommon/sources/directive b/jni/libucommon/sources/directive
new file mode 100644
index 0000000..240d8fc
--- /dev/null
+++ b/jni/libucommon/sources/directive
@@ -0,0 +1,3 @@
+version: 1.1
+directory: commoncpp
+filename: ucommon-6.0.7.tar.gz