Fix rspec 3 deprecation warnings

Using rspec 3 we get some deprecation warnings. This change fixes those, and bumps the gem version and rspec dependencies.

```
Deprecation Warnings:

`failure_message_for_should_not` is deprecated. Use `failure_message_when_negated` instead. Called from /usr/local/opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/resque_spec-0.15.0/lib/resque_spec/matchers.rb:118:in `block in <top (required)>'.
`failure_message_for_should_not` is deprecated. Use `failure_message_when_negated` instead. Called from /usr/local/opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/resque_spec-0.15.0/lib/resque_spec/matchers.rb:98:in `block in <top (required)>'.

`failure_message_for_should` is deprecated. Use `failure_message` instead. Called from /usr/local/opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/resque_spec-0.15.0/lib/resque_spec/matchers.rb:114:in `block in <top (required)>'.
`failure_message_for_should` is deprecated. Use `failure_message` instead. Called from /usr/local/opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/resque_spec-0.15.0/lib/resque_spec/matchers.rb:94:in `block in <top (required)>'.
```