Run specs against the latest release of 2.4

.travis.yml

@@ -365,7 +365,7 @@ env:
   - &rubyspec
     name: Check ruby/spec version guards on Ruby 2.4
     language: ruby
-    rvm: 2.4.6
+    rvm: 2.4.9
     before_install:
     install:
     before_script: chmod -R u+w spec/ruby

spec/README.md

@@ -66,7 +66,7 @@ See `spec/ruby/CONTRIBUTING.md` for more documentation about guards.
 To verify specs are compatible with older Ruby versions:
 ```
 cd spec/ruby
-$RUBY_MANAGER use 2.4.6
+$RUBY_MANAGER use 2.4.9
 ../mspec/bin/mspec -j
 ```