Change to using eql true/false syntax

Tests in Travis were red :-1:

```
1) Puppet::Type::Mongodb_replset::ProviderMongo#exists? when the replicaset does not exist returns false
     Failure/Error: resource.provider.exists?.should be_false
       expected false to respond to `false?`
```

For some reason the be_false syntax wasn't working? I've changed them to eql true and it's all green now...

Tests are now green :+1: