Fix legacy Ruby versions

The current codebase doesn't work correctly with Ruby <= 1.9 because even though @jcamenisch did add some good Ruby 1.8.7 conditions into the codebase, [the `INTERPOLATED_SHELL_COMMAND` Regexp](https://github.com/bkeepers/dotenv/blob/837021444301b355f6be9878436adcb370539ca1/lib/dotenv/environment.rb#L29-L37) is still defined using a lookbehind syntax that isn't supported by older versions of Ruby. This has caused some of [the latest Travis builds](https://travis-ci.org/bkeepers/dotenv/jobs/13840719) fail with a `SyntaxError`.

This pull request resolves this problem by moving the definition of the `INTERPOLATED_SHELL_COMMAND` Regexp into a completely separate .rb file that is only required when modern versions of Ruby are used.

With this pull request, all tests are now [passing on all environments!](https://travis-ci.org/metavida/dotenv/builds/14424488)