Use travis_wait for test-spec as well

rubyspec seems very slow or getting stuck on s390-linux.

https://app.travis-ci.com/github/ruby/ruby/jobs/617916019
No output has been received in the last 10m0s, this potentially
indicates a stalled build or something wrong with the build itself.

.travis.yml

@@ -135,7 +135,7 @@ before_script:
 script:
   - $SETARCH make -s test
   - ../tool/travis_wait.sh $SETARCH make -s test-all RUBYOPT="-w"
-  - $SETARCH make -s test-spec
+  - ../tool/travis_wait.sh $SETARCH make -s test-spec
 
 # We want to be notified when something happens.
 notifications: