Make installing deps more stable

The old syntax works with bash, but not with zsh. The new syntax works with both.

CONTRIBUTORS.rst

@@ -221,6 +221,7 @@ Contributors
 * Tom Hendrikx
 * Ralph Jacobs
 * Wietze Helmantel
+* Patrick Gerken
 
 Translators
 ===========

docs/contributing/developing.rst

@@ -31,7 +31,7 @@ With your preferred virtualenv activated, install the Wagtail package in develop
 
 .. code-block:: console
 
-    $ pip install -e .[testing,docs] -U
+    $ pip install -e '.[testing,docs]' -U
 
 Install Node through nvm (optional):