Update factory-boy requirement from ==2.11.* to ==2.12.* (#356)

Updates the requirements on [factory-boy](https://github.com/FactoryBoy/factory_boy) to permit the latest version.
- [Release notes](https://github.com/FactoryBoy/factory_boy/releases)
- [Changelog](https://github.com/FactoryBoy/factory_boy/blob/master/docs/changelog.rst)
- [Commits](FactoryBoy/factory_boy@2.11.0...2.12.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements/test.txt

@@ -2,7 +2,7 @@ pytest==3.4.*
 pytest-cov==2.5.*
 pytest-django==3.2.*
 pytest-pythonpath==0.7.*
-factory-boy==2.11.*
+factory-boy==2.12.*
 mixer==6.0.*
 isort==4.3.*
 flake8==3.5.*