add Gemfile

add Gemfile for Bundler

----

When I execute this app, I caught encoding error.

<pre>
C:\Users\kaakaa_hoe\Documents\GitHub\RubyStackMemos>bundle exec ruby stack_memos
.rb
stack_memos.rb:52: invalid multibyte char (US-ASCII)
stack_memos.rb:52: invalid multibyte char (US-ASCII)
stack_memos.rb:52: syntax error, unexpected $end, expecting keyword_end
                @message = "ERROR: 辟｡蜉ｹ縺ｪ繝代Λ繝｡繝ｼ繧ｿ繝ｼ繧呈...
</pre>

I think that this error is caused by misplaced "magic comment (i.e. '# encoding: utf-8' )".
When I replace magic comment to first line this app work.