Bump up the ubuntu version used in CI to bionic.

VRF support was introduced in ubuntu bionic, while it's missing in Xenial.

This also introduces a change in the behaviour of nc command.
On one hand, it requires a new line to send the buffer on the other side,
on the other it hangs waiting for new input.

To address this, a timeout was introduced to avoid the tests to hang,
plus the buffer sent is terminated with a new line character.

Signed-off-by: Federico Paolinelli <fpaoline@redhat.com>

.travis.yml

@@ -1,6 +1,6 @@
 language: go
 sudo: required
-dist: xenial
+dist: bionic
 
 go:
   - 1.13.x