Gemspec cleanup

Hi, I would like to sugest some changes.
In the moment to use this gem, I need to use `gem 'Blockcypher Ruby SDK', github: 'blockcypher/ruby-client'` in my Gemfile. I think the name `gem 'blockcypher-ruby', github: 'blockcypher/ruby-client'` is more idiomatic, because major of the gems use the downcase form separated by hyphen or underscore. I also changed the `files` field to lookup the files dynamically, and removed `lib/client.rb` which doesn't exist.