Exclude CodeCov on Ruby < 2.3.0

It appears that the codecov gem now uses the &. operator,
which appeared in Ruby 2.3.

Gemfile

@@ -2,7 +2,7 @@ require File.expand_path("on_what", File.dirname(__FILE__))
 source "https://rubygems.org"
 
 # Travis-only dependencies go here
-if on_travis? && !on_1_8? && !on_rubinius?
+if on_travis? && !on_less_than_2_3? && !on_rubinius?
   group :test do
     gem "codecov", ">= 0.0.9", :require => false
   end

on_what.rb

@@ -24,3 +24,7 @@ end
 def on_less_than_2_0?
   RUBY_VERSION < "2.0.0"
 end
+
+def on_less_than_2_3?
+  RUBY_VERSION < "2.3.0"
+end