Change deprecated features

For RSPEC 3 one needs to expose failure_message and failure_message_when_negated,
failure_message_for_should and failure_message_for_should_not are deprecated.

Also updated the specs to use the new expect(subject).to syntax.
Merge branch 'make-gem-rspec3-compatible' into merge_forks

Conflicts:
	lib/rspec-xml/xml_matchers/have_xpath.rb
	spec/rspec-xml/xml_matchers/have_xpath_matcher_spec.rb
Merge branch 'master' into merge_rspec3

Conflicts:
	lib/rspec-xml/xml_matchers/have_xpath/matcher.rb
	lib/rspec-xml/xml_matchers/have_xpath/text_matcher.rb
	spec/features/steps/have_path_steps.rb
	spec/rspec-xml/xml_matchers/have_xpath/matcher_spec.rb
	spec/rspec-xml/xml_matchers/have_xpath/text_matcher_spec.rb
	spec/rspec-xml/xml_matchers/have_xpath_matcher_spec.rb
Cleanup after merge
Remove deprecation warnings
Add travis control file
Add gem and travis badges to Readme