cJSON `make install` doesn't build, so build it first.

travis-install.sh

@@ -5,6 +5,7 @@ if [ "$TRAVIS_OS_NAME" == "linux" ]; then
 	sudo apt-get update -qq
 	sudo apt-get install -y debhelper libc-ares-dev libssl-dev libwrap0-dev python-all python3-all uthash-dev xsltproc docbook-xsl libcunit1-dev
 	git clone https://github.com/DaveGamble/cJSON
+	make -C cJSON
 	sudo make -C cJSON install
 fi