Add support for running without Bundler
Add support for running without Bundler

If we install dummer by "gem install dummer" and run dummer by
"dummer" (no "bundle exec"), the following error is reported:

    % dummer
    fatal: not a git repository (or any of the parent directories): .git
    Could not find gem 'rspec' in any of the gem sources listed in your Gemfile.
    Run `bundle install` to install missing gems.

Because bin/dummer uses Gemfile in the dummer-0.4.0/ directory when we
don't run "dummer" without "bundle exec". It causes the "Could not
find gem ..." error message.

Then Gemfile reads "dummer.gemspec" and "dummer.gemspec" uses "git
ls-files". It causes the "fatal: not a git repository ..." error
message.

sonots/dummer