Run specs against 2.3.7 to ensure version guards are correctly added

* See [Feature #15004].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64502 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -63,6 +63,11 @@ script:
   - "make -s test TESTOPTS=--color=never"
   - "make -s $JOBS test-all -o exts TESTOPTS='-q --color=never --job-status=normal' RUBY_FORCE_TEST_JIT=1"
   - "make -s $JOBS test-spec MSPECOPT=-j"
+  - echo "Running ruby/spec on 2.3.7 to ensure version guards are correctly added"
+  - rvm install 2.3.7
+  - cd ../spec/ruby
+  - chmod -R u+w .
+  - ../mspec/bin/mspec -j .
 
 # Branch matrix.  Not all branches are Travis-ready so we limit branches here.
 branches: