Fix an exception when the DeprecationFormatter is initialized with an IO stream that doesn't respond to :path.

When using spring, sometimes @file points to <IO:/dev/null>, which doesn't respond to "path" and raises an exception. This fixes that problem.

Please let me know if you'd like me to change anything or add any more color. Thanks!

For reference, here is the stack trace that this fixes:

```
/Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/formatters/deprecation_formatter.rb:212:in `summarize': undefined method `path' for #<IO:/dev/null> (NoMethodError)
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/formatters/deprecation_formatter.rb:130:in `deprecation_summary'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/formatters/deprecation_formatter.rb:41:in `deprecation_summary'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/reporter.rb:134:in `block in notify'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/reporter.rb:133:in `each'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/reporter.rb:133:in `notify'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/reporter.rb:115:in `finish'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/reporter.rb:55:in `report'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:107:in `run_specs'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:85:in `run'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:69:in `run'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:37:in `invoke'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rspec-core-3.1.7/exe/rspec:4:in `<top (required)>'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:241:in `load'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:241:in `block in load'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:232:in `load_dependency'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:241:in `load'
        from /Users/kevin/programming/boundless/www/bin/rspec:16:in `<top (required)>'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:241:in `load'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:241:in `block in load'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:232:in `load_dependency'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:241:in `load'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from /Users/kevin/.rbenv/versions/2.1.0/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from -e:1:in `<main>'
```