Configure Bundler to validate task dependencies

[ashmaroli]

@pyrmont This should let one use our rake tasks without having to prepend bundle exec.

To test:

• Test-1: Delete any existing Gemfile.lock and run rake changelog:insert
• Test-2: Rename vendor directory (or wherever Bundler is configured to install) to something else and try running rake

Bundler should inform you if about bundle install if one of the dependencies have not been installed.

[pyrmont]

Thanks @ashmaroli! This looks good!

[ashmaroli]

You're welcome. Let me know if it works for your use-case.