[DOC] Fix Travis-CI condition

"Not document only" should be honored always.  Also simplified repo
and branch conditions

.travis.yml

@@ -11,7 +11,9 @@
 # https://github.com/ruby/ruby/wiki/CI-Servers#travis-ci
 
 # We enable Travis on the specific branches or forked repositories here.
-if: (repo = ruby/ruby AND (branch = master OR branch =~ /^ruby_\d_\d$/)) OR repo != ruby/ruby OR commit_message !~ /\[DOC\]/
+if: >-
+  (repo != ruby/ruby OR branch = master OR branch =~ /^ruby_\d_\d$/)
+  AND (commit_message !~ /\[DOC\]/)
 
 language: c