Work around bug in 1.8.7's `Method#to_proc`.

I considered keeping the `Method#to_proc` implementation
for other rubies (1.9+), assuming it would be faster,
but benchmarks reveal it is in fact _slower_.
Merge pull request #641 from rspec/fix-matcher-definition-1.8.7

Work around bug in 1.8.7's `Method#to_proc`.