Get build passing under rubinius. Add to CI.

.travis.yml

@@ -5,5 +5,6 @@ env:
   - RSPEC_VERSION=">=3.1.0"
 rvm:
   - jruby
+  - rbx-2
   - 1.9.3
   - 2.0.0

examples/integration/failing_messages_spec.rb

@@ -2,6 +2,7 @@
 require 'open3'
 
 describe "Failing Messages" do
+  skip_natural_assertions_if_not_supported
 
   IOS = Struct.new(:out, :err)
 

examples/integration/then_spec.rb

@@ -11,6 +11,7 @@ def to_bool
 
 describe "Then" do
   context "empty thens with natural assertions" do
+    skip_natural_assertions_if_not_supported #<--we can't detect void statements
     Then { }
   end
   context "thens to_bool/true will pass" do

examples/minitest_helper.rb

@@ -1,4 +1,3 @@
-
 module GivenAssertions
   def given_assert(cond)
     assert cond
@@ -22,5 +21,14 @@ def given_assert_raises(error, pattern=//, &block)
   end
 end
 
+module NaturalAssertionControl
+  def skip_natural_assertions_if_not_supported
+    if !Given::NATURAL_ASSERTIONS_SUPPORTED
+      Given { skip "This test requires a Ruby runtime with full natural assertions support." }
+    end
+  end
+end
+
 Minitest::Spec.send(:include, GivenAssertions)
 Minitest::Test.send(:include, GivenAssertions)
+include NaturalAssertionControl