Fix issue #64

This fixes the data corruption problem seen in [this comment](https://github.com/colszowka/simplecov/issues/64#issuecomment-8550133) of issue #64.

Finished tests in 4.232778s, 27.4052 tests/s, 33.3115 assertions/s.

116 tests, 141 assertions, 0 failures, 0 errors, 0 skips
/Users/jshraibman/.rvm/rubies/ruby-1.9.3-p125/bin/ruby -S bundle exec cucumber 
Using the default profile...
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44 scenarios (44 passed)
217 steps (217 passed)
1m59.931s