changed rspec method names to remove deprecation warnings

This fixes these deprecation warnings:

```
Deprecation Warnings:

`failure_message_for_should_not` is deprecated. Use `failure_message_when_negated` instead. Called from /Users/seandevine/Projects/federatedfreightways/equivalent-xml/lib/equivalent-xml/rspec_matchers.rb:57:in `block in <module:RSpecMatchers>'.

`failure_message_for_should` is deprecated. Use `failure_message` instead. Called from /Users/seandevine/Projects/federatedfreightways/equivalent-xml/lib/equivalent-xml/rspec_matchers.rb:53:in `block in <module:RSpecMatchers>'.
```