Added Unix instructions to upgrade guide from FactoryGirl to FactoryBot

[roooodcastro]

The new 4.9.0 version issues a deprecation warning when I run my tests, and tell me to go to UPGRADE_FROM_FACTORY_GIRL.md to see how to upgrade.

This page contains an example grep + sed command to automatically do the job for you. However it didn't work for me running Ubuntu 16.04.

I then fixed the command to work on Unix and made this patch to add the unix example command.

[jbcden]

For what it's worth I also found that I had to use the fix here for macOS 10.12.5. The suggested returned no results.

[jbcden]

Actually just tweaked this again and managed to get the macOS version to work with:

grep -e FactoryBot **/**/*.rake **/**/*.rb -s -l

This seems to also work on Ubuntu 😄

[composerinteralia]

Thanks for this PR! The example we gave was really intended to be an example, not necessarily a perfect solution that would work for everyone. We have had lots of PRs suggesting alternatives (#1070, #1075, #1084, #1095, #1102), but to reduce churn on this file I have decided to close these issues. I appreciate your help!