Fix wrong variable reports in it_is_asserted_by

[seikichi]

Consider the following test:

context do
  let(:blk) { [1, 2, 3] }
  it_is_asserted_by { blk == %w(a b c) }
end

The above test outputs the following report (blk is wrong!):

Failure/Error: Unable to find matching line from backtrace
          it_is_asserted_by { blk == %w(a b c) }
                              |   |
                              |   false
                              #<Proc:0x...>

This is because RSpec::PowerAssert.evaluate_example sends
wrong assertion proc pr to PowerAssert::Context.
The PowerAssert::Context calls
@assertion_proc.binding.eval('local_variables')
and uses the local variables to create reports.
Thus, PowerAssert::Context uses wrong local variables to create report.

This commit fixes the above problem by wrapping
the assertion proc pr in blk binding context.