Gem / include naming

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Currently I have to do this:

gem 'slack-api', github: 'aki017/slack-ruby-gem', require: 'slack'

The gem is called slack-api, but require slack, and the project name is called slack-ruby-gem. I think you want to rename everything into slack-api.

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In #31 I added a slack-api.rb so this fixes the need for require above.

The last step is to rename this repo from slack-ruby-gem to slack-api.