Revert "CI: remove sudo and kubuntu backports"

This reverts commit a6b364e.

Author: termoshtt

.travis.yml

@@ -1,4 +1,5 @@
 language: rust
+sudo: required
 
 rust:
   - stable
@@ -19,6 +20,9 @@ script:
 
 addons:
   apt:
+    sources:
+      - kubuntu-backports
+
     packages:
       - cmake
       - gfortran