Should fail gracefully if keys aren't set

[eshamow]

Sample output:

➜  baseboxes git:(master) march user
/usr/local/lib/ruby/gems/2.3.0/gems/march-audit-0.1.0/lib/march/client.rb:26:in `fetch': key not found: "GITHUB_TOKEN" (KeyError)
        from /usr/local/lib/ruby/gems/2.3.0/gems/march-audit-0.1.0/lib/march/client.rb:26:in `client'
        from /usr/local/lib/ruby/gems/2.3.0/gems/march-audit-0.1.0/bin/march:33:in `user'
        from /usr/local/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor/command.rb:27:in `run'
        from /usr/local/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor/invocation.rb:126:in `invoke_command'
        from /usr/local/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor.rb:369:in `dispatch'
        from /usr/local/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor/base.rb:444:in `start'
        from /usr/local/lib/ruby/gems/2.3.0/gems/march-audit-0.1.0/bin/march:39:in `<top (required)>'
        from /usr/local/bin/march:22:in `load'
        from /usr/local/bin/march:22:in `<main>'