Port data_objects tests to rspec3

This let's most tests pass with rspec3, except for:
```
Failures:

  1) DataObjects::Pooling should wake up the scavenger thread when exiting
     Failure/Error: DataObjects::Pooling.scavenger?.should be_false
       expected false to respond to `false?`
     # ./spec/pooling_spec.rb:152:in `block (2 levels) in <top (required)>'

And there are some warnings:
```
Deprecation Warnings:

Using `should_receive` from rspec-mocks' old `:should` syntax without explicitly enabling the syntax is deprecate
d. Use the new `:expect` syntax or explicitly enable `:should` instead. Called from /usr/obj/ports/data_objects-0
.10.14-ruby21/data_objects-0.10.14/spec/transaction_spec.rb:7:in `block (2 levels) in <top (required)>'.

Using `should` from rspec-expectations' old `:should` syntax without explicitly enabling the syntax is deprecated
. Use the new `:expect` syntax or explicitly enable `:should` instead. Called from /usr/obj/ports/data_objects-0.
10.14-ruby21/data_objects-0.10.14/spec/command_spec.rb:15:in `block (3 levels) in <top (required)>'.
```

Untested with rspec2, nor has the dependency in the Gemfile been bumped.