Be friendlier when upgrading apps with an old boot.rb.
Be friendlier when upgrading apps with an old boot.rb.

If Rails doesn't respond to vendor_rails?, abort with an error asking
the user to run 'rake rails:update'.

Signed-Off-By: Michael Koziarski <17b9e1c64588c7fa6419b4d29dc1f4426279ba01@koziarski.com>

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