Remove #by used with the #not_to method

It's explain https://github.com/rspec/rspec-expectations/commit/953e3b36217ffa491a75bc327e878382dbc22941
and it cause error 'expect { }.not_to change { }.by()` is not supported'
Merge pull request #4 from sekarasiewicz/master

Remove #by used with the #not_to method