Provision with Chef version 11.8.0

To merge when all Travis `ci_environment` cookbooks are runnable with Chef 11.

**TODO List:**
*Let's report here problems (and fixes) around Chef 11 compatibility*

* Riak cookbook (template compilation error), see travis-ci/travis-ci#1090 about possible evolutions of this cookbook.
* We should remove the extra `include_attribute` statements needed with Chef 10. See travis-ci/travis-cookbooks@f328c6ba74ea792a31b4e5514cb2df5002c11ed6 and comment below.