(maint) remove rspec 2.10 method call

Rspec 2.10 supports a named subject by calling subject with the desired
name, but this isn't backwards compatible with Rspec 2.9. Since we're
not really exploiting the rspec behavior around the subject it is
easiest to change this to use a simple `let` block.
Merge branch 'maint-rspec_2.9_let'