dev-env: add Gemfile, ignore Gemfile.lock and tmp dir

Since the gemspec contains all appropriate development dependencies,
defer to the gemspec in a Gemfile to add bundler support without requiring
devs to use bundler; ignore associated lock file to prevent devs from
accidentally adding it.

Ignore tmp directory to prevent accidental adding and polution of the git
status, since running the tests will add redis sources here.

[ci skip]