Failing when using Dir.mktmpdir in spec

Hi, I am getting the following error when I tried to use the `fivemat` formatter (Win 7, Ruby 2.2+, Rspec 3.7+)
```ruby
Failure/Error:
       Dir.mktmpdir do |dir|
...
...
NoMethodError:
       undefined method `mktmpdir' for Dir:Class
```
The `specs` run fine with the default formatter, and I am not able to find any namespace conflicts.
Any idea, anyone?
