Run redcarpet in redcarpet_bin_test with the interpreter the test started with
Run redcarpet in redcarpet_bin_test with the interpreter the test started with

This fixes test failure for Ruby 2.5 when Ruby 2.3 is the default installed
version.

kaneshin/redcarpet,fukayatsu/redcarpet,fukayatsu/redcarpet,kaneshin/redcarpet,increments/greenmat,vmg/redcarpet,fukayatsu/redcarpet,kaneshin/redcarpet,increments/greenmat,vmg/redcarpet,increments/greenmat,increments/greenmat,fukayatsu/redcarpet,kaneshin/redcarpet,vmg/redcarpet,vmg/redcarpet