Enable Travis on the specific branches or forked repositories.

Align the Travis enabling timing with GitHub Actions.

For the syntax, see <https://docs.travis-ci.com/user/conditions-v1>.
We use `repo` syntax rather than `fork = true/false` syntax to show a general usage in any repositories on GitHub.
The non-forked repo is not always a primary repo in any GitHub repositories.

[Bug #17936]

.travis.yml

@@ -203,11 +203,8 @@ script:
   - $SETARCH make -s test-spec MSPECOPT=-ff # not using `-j` because sometimes `mspec -j` silently dies
   - $SETARCH make -s -o showflags leaked-globals
 
-# Branch matrix.  Not all branches are Travis-ready so we limit branches here.
-branches:
-  only:
-    - master
-    - /^ruby_\d_\d$/
+# We enable Travis on the specific branches or forked repositories here.
+if: (repo = ruby/ruby AND (branch = master OR branch =~ /^ruby_\d_\d$/)) OR repo != ruby/ruby
 
 # We want to be notified when something happens.
 notifications: