Add a syntax test to help support multiple Rubies

This is no real gain on Travis, which runs the full test suite
against two Rubies. It does catch a few classes of error when
running the test suite locally.

test/syntax_test.rb

@@ -0,0 +1,32 @@
+require 'test_helper'
+
+describe "Syntax check" do
+  project_root = Pathname.new(File.expand_path("#{File.dirname(__FILE__)}/../"))
+  backend_files = Dir[project_root.join('**', '*.rb')].reject{|f| f.match %r{/Casks/}}
+  %w{1.8 1.9 2.0 2.1}.each do |major_version|
+    describe "under Ruby #{major_version}" do
+      interpreter = Pathname.new("/System/Library/Frameworks/Ruby.framework/Versions/#{major_version}/usr/bin/ruby")
+      interpreter = Pathname.new(Dir["#{ENV['HOME']}/.rbenv/versions/#{major_version}*/bin/ruby"].first.to_s)  unless interpreter.exist?
+      interpreter = Pathname.new(Dir["#{ENV['HOME']}/.rvm/rubies/ruby-#{major_version}*/bin/ruby"].first.to_s) unless interpreter.exist?
+      interpreter = Pathname.new(Dir["/usr/local/bin/ruby{-,}#{major_version}*"].first.to_s)                   unless interpreter.exist?
+      interpreter = Pathname.new(Dir["/opt/local/bin/ruby{-,}#{major_version}*"].first.to_s)                   unless interpreter.exist?
+      flags = %w{-c}
+      flags.unshift '--disable-all' if major_version.to_f > 1.8   # perf
+      backend_files.each do |file|
+        it "#{file} is valid Ruby" do
+          skip unless interpreter.exist?
+
+          # Reason for this hack is unknown. Travis appears to freeze,
+          # but only in one section of the build matrix.
+          skip if ENV.key?('TRAVIS_JOB_ID')     and
+                  RUBY_VERSION.match(%r{^1\.8})
+
+          args = flags + [ '--', file ]
+          shutup do
+            raise SyntaxError.new("#{file} failed syntax check") unless system(interpreter, *args)
+          end
+        end
+      end
+    end
+  end
+end