Use `bundle config` instead of a deprecated flag in an error message.

[tatsuyafw]

What was the end-user or developer problem that led to this PR?

When trying to install gems to the system RubyGems and it fails, bundler shows a message as follows:

$ bundle install
# snip
Your user account isn't allowed to install to the system RubyGems.
  You can cancel this installation and run:

      bundle install --path vendor/bundle

  to install the gems into ./vendor/bundle/, or you can enter your password
  and install the bundled gems to RubyGems using sudo.

But the --path flag is deprecated, bundle install --path vendor/bundle shows:

$ bundle install --path vendor/bundle
[DEPRECATED] The `--path` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bundle config set path 'vendor/bundle'`, and stop using this flag
# snip

What is your fix for the problem, implemented in this PR?

This PR changes the message to show bundle config set --local path instead of the deprecated --path flag.

[deivid-rodriguez]

@bundlerbot r=colby-swandale

Yes, thanks!

[ghost]

Build succeeded

• continuous-integration/travis-ci/push
• windows (2.4.x)
• windows (2.5.x)
• windows (2.6.x)

[tatsuyafw]

Thank you for reviewing!