run valgrind in Travis CI

Because this runs apt-get, it will not work as-is for OSX. So when
we have OSX in Travis, we will have to wrap this somehow. See #250.

Closes #222.
diff --git a/dev.makefile b/dev.makefile
index e289266..d288b16 100644
--- a/dev.makefile
+++ b/dev.makefile
@@ -26,6 +26,9 @@
 	python3.4 amalgamate.py
 	cd dist; gcc -I. -c jsoncpp.cpp
 
+valgrind:
+	valgrind --error-exitcode=42 --leak-check=full ./build/debug/src/test_lib_json/jsoncpp_test
+
 clean:
 	\rm -rf *.gz *.asc dist/