Change Makefile to use sed syntax that will work for both GNU and BSD…

… sed (redis#719)

* Use sed syntax that will work with both GNU and BSD sed

Makefile

@@ -15,5 +15,5 @@ testdata/redis:
 	wget -qO- https://github.com/antirez/redis/archive/unstable.tar.gz | tar xvz --strip-components=1 -C $@
 
 testdata/redis/src/redis-server: testdata/redis
-	sed -i 's/libjemalloc.a/libjemalloc.a -lrt/g' $</src/Makefile
+	sed -i.bak 's/libjemalloc.a/libjemalloc.a -lrt/g' $</src/Makefile
 	cd $< && make all