Test BASERUBY: Ruby 1.9.3 on Travis

We have no clear assertion or check of BASERUBY requirement.
I want to make the current situation more explicit.

I'm NOT saying we should support Ruby 1.9.3 here,
but I'm just checking the situation as per 05bc14d81a.

FYI, at this moment Ruby 1.8.7 did not work with this Travis config, like:
https://travis-ci.org/k0kubun/ruby/builds/538459100

.travis.yml

@@ -314,6 +314,14 @@ env:
     # -j randomly hangs.
     script: ruby -C spec/ruby ../mspec/bin/mspec .
 
+  - &baseruby
+    name: "BASERUBY: Ruby 1.9.3"
+    <<: *linux
+    <<: *make-test-only
+    dist: trusty # xenial no longer has ruby-1.9.3
+    language: ruby
+    rvm: 1.9.3
+
   - &x86_64-darwin17
     name: x86_64-darwin17
     <<: *osx
@@ -342,6 +350,7 @@ matrix:
     - <<: *CALL_THREADED_CODE
     - <<: *NO_THREADED_CODE
     - <<: *rubyspec
+    - <<: *baseruby
   allow_failures:
     - name: -fsanitize=address
     - name: -fsanitize=memory