Run specs on latest Ruby 2.3, which includes taint fixes

* The 2.3 alias in RVM seems to resolve to 2.3.7.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65390 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -160,7 +160,7 @@ matrix:
           LDFLAGS=-Wno-unused-command-line-argument)
     - os: linux
       language: ruby
-      rvm: 2.3
+      rvm: 2.3.8
       name: Running ruby/spec on 2.3 to ensure version guards are correctly added
       addons:
         apt: