travis: upgrade distribution to Ubuntu 20.04

The current default Travis default is 16.04 which is obsolete.

From thesofproject/linux#2402

> Everyone uses 20.04, period. SOF CI also uses 20.04.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

.travis.yml

@@ -6,6 +6,8 @@
 # yaml merge-expand .travis.yml exp.yml && diff -b -u .travis.yml exp.yml
 
 language: c
+# Ubuntu 20.04 LTS
+dist: focal
 
 git:
   depth: false