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/.dockerignore b/.dockerignore
index 1b2211d..ef67c26 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1 +1,2 @@
 build*
+.git