travis: Test in Ubuntu 18.04 container

- Install gnuradio from the new official PPA repository
  https://launchpad.net/~gnuradio/+archive/ubuntu/gnuradio-releases

Change-Id: Iaa8813ab0ae91b667285ab3ef42ffaa279e3a885
diff --git a/.travis.yml b/.travis.yml
index 182e006..437e66e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,6 +9,7 @@
 #  - DOCKERFILE=tests/dockerfiles/Ubuntu_16_04_nolibosmo.docker IMGNAME=ubu16.04-grgsm-nolibosmo
   - DOCKERFILE=tests/dockerfiles/Kali.docker IMGNAME=kali-grgsm
 #  - DOCKERFILE=tests/dockerfiles/Fedora_26.Dockerfile IMGNAME=fedora26-grgsm
+  - DOCKERFILE=tests/dockerfiles/Ubuntu_18_04_gnuradio_ppa.docker IMGNAME=ubuntu_18_04_gnuradio_ppa
 
 
 services: