Outdated documentation for dev environment

[hodgesmr]

The developer dependency documentation appears to be out of date:

Axelrod/docs/how-to/contributing/setting_up_the_environment.rst

Lines 7 to 17 in 3fc68ed

	All development dependencies can be installed by running::
	$ pip install -r requirements/development.txt
	It is recommended to do this using a virtual environment tool of your choice.
	For example, when using the virtual environment library :code:`venv`::
	$ python -m venv axelrod_development
	$ source axelrod_development/bin/activate
	$ pip install -r requirements/development.txt

The file requirements/development.txt doesn't appear to exist. Looks like it was removed in #1440