Remove lib/benchmark/helpers.rb from Manifest file.

This file has already been removed from https://github.com/JuanitoFatas/benchmark-ips/commit/bc44856d5145fdebe9e07b9dbb4bc27cfacb0f6a.
Merge pull request #19 from JuanitoFatas/fix/build-gem

Remove lib/benchmark/helpers.rb from Manifest file.