Fix rubocop offenses in generated files

[nicoalonsop]

When scaffolding a new model the generator generates the factory with rubocop offenses (using default rubocop configuration for rails).

Fixed offenses:

• Style/StringLiterals
• Style/FrozenStringLiteralComment

[composerinteralia]

Thanks for the PR! thoughtbot uses https://github.com/thoughtbot/guides/blob/master/style/ruby/.rubocop.yml as the default config (it prefers double quotes and disables the frozen string literal comments), so I am going to close this.

You can read more about our choice of single quotes, and our choice of disabling the frozen string literal comment in the guides repo.