Fix uninitialized constant Tempfile in dns_spec.rb

[Watson1978]

The tempfile library was implicitly loaded when running the entire test suite, but running spec/dns_spec.rb alone resulted in a NameError. Added require 'tempfile' to ensure the test can be executed independently.

$ rspec spec/dns_spec.rb

DNS
  connects to valid domains
  fires on_resolve_failed for invalid domains
  resolve localhost even though hosts is empty (FAILED - 1)
  resolve missing localhost even though hosts entries exist (FAILED - 2)

Failures:

  1) DNS resolve localhost even though hosts is empty
     Failure/Error:
       Tempfile.open("empty") do |file|
         expect( Coolio::DNSResolver.hosts("localhost", file.path)).to eq @preferred_localhost_address
       end

     NameError:
       uninitialized constant Tempfile
     # ./spec/dns_spec.rb:46:in 'block (2 levels) in <top (required)>'
...

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