Fix rubygems LoadError.

[junaruga]

I got below error on my local environment.

$ bundle exec rake test
Traceback (most recent call last):
  1: from /usr/local/ruby-2.5.1/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
/usr/local/ruby-2.5.1/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require': cannot load such file -- ubygems (LoadError)
rake aborted!
Command failed with status (1)
/home/jaruga/git/sinatra/vendor/bundle/ruby/2.5.0/gems/rake-12.3.1/exe/rake:27:in `<top (required)>'
/usr/local/ruby-2.5.1/bin/bundle:23:in `load'
/usr/local/ruby-2.5.1/bin/bundle:23:in `<main>'
Tasks: TOP => test
(See full trace by running task with --trace)

This logic is old when rubygem was not included as a stdlib, isn't it?
I removed this logic :)
How do you think?

[dentarg]

There is already a fix for this at #1436

[junaruga]

@dentarg thanks for the info. I would close this ticket.