travis: remove sudo: required

The sudo keyword will be fully deprecated.

https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
Signed-off-by: Christian Berendt <berendt@betacloud-solutions.de>

.travis.yml

@@ -1,4 +1,3 @@
-sudo: required
 language: python
 python:
   - "3.6"
@@ -10,4 +9,4 @@ install:
   - git clone https://github.com/ansible/galaxy-lint-rules
 script:
   - pytest -vvvv library/
-  - for i in $(ls -1 roles/); do ansible-lint -x 204 -v -r galaxy-lint-rules/rules roles/$i/ ; done
+  - for i in $(ls -1 roles/); do ansible-lint -x 204 -v -r galaxy-lint-rules/rules roles/$i/ ; done