Add MRI 2.4 plus frozen string literal check to .travis.yml

Author: pat

.travis.yml

@@ -7,6 +7,7 @@ rvm:
   - 2.1
   - 2.2
   - 2.3.0
+  - 2.4.1
   - jruby-18mode
   - jruby-19mode
   - jruby-9.0.5.0
@@ -16,6 +17,9 @@ gemfile: ".travis/Gemfile"
 
 sudo: false
 
+before_script:
+- if (ruby -e "exit RUBY_VERSION.to_f >= 2.4"); then export RUBYOPT="--enable-frozen-string-literal"; fi; echo $RUBYOPT
+
 env:
   global:
     - VERBOSE=true