add to travis checking of valid documentation

.travis.yml

@@ -9,3 +9,4 @@ script:
   # the "yast-travis-ruby" script is included in the base yastdevel/ruby image
   # see https://github.com/yast/docker-yast-ruby/blob/master/yast-travis-ruby
   - docker run -it -e TRAVIS=1 -e TRAVIS_JOB_ID="$TRAVIS_JOB_ID" yast-bootloader-image yast-travis-ruby
+  - docker run -it yast-bootloader-image rake check:doc

Rakefile

@@ -4,4 +4,5 @@ Yast::Tasks.configuration do |conf|
   # lets ignore license check for now
   conf.skip_license_check << /.*/
   conf.install_locations["doc/autodocs"] = conf.install_doc_dir
+  conf.documentation_minimal = 50 # TODO: improve it, at least do not get worse
 end