CI/travis/make_linux: install deb package as final test

The fact that the `deb` packages build doesn't mean they install, so as a
final test, use `dpkg` to install them
`dpkg` is preferred because `apt` may choose to install `libiio` from the
apt sources if it prefers it over local `deb` files.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>

CI/travis/make_linux

@@ -6,6 +6,7 @@ handle_default() {
 	cd $TRAVIS_BUILD_DIR/build
 	cmake -DENABLE_PACKAGING=ON -DDEB_DETECT_DEPENDENCIES=ON ..
 	make && make package
+	sudo dpkg -i libiio*.deb
 }
 
 handle_centos() {