Update from FactoryGirl to FactoryBot

[isabel22]

As the gem has been updated and changed its name from FactoryGirl to FactoryBot, I decided to make the the change as suggested in the documentation of thoughtbot.

After updating, I ran rspec and noticed that there was an error:

DEPRECATION WARNING: Static attributes will be removed in FactoryBot 5.0. Please use dynamic attributes instead by wrapping the attribute value in a block.

To fix this warning and automatically update from static attributes to dynamic attributes, I installed locally rubocop-rspec and ran:

rubocop \
  --require rubocop-rspec \
  --only FactoryBot/AttributeDefinedStatically \
  --auto-correct

This warning was fixed in the second commit.

This PR contributes to #10.