Eliminate duplication of dependencies in tox.ini

Fixes #2317

tox.ini

@@ -10,6 +10,7 @@ exclude = wagtail/project_template/*
 max-line-length = 120
 
 [testenv]
+install_command = pip install -e ".[testing]" -U {opts} {packages}
 commands =
     elasticsearch: coverage run runtests.py wagtail.wagtailsearch --elasticsearch
     noelasticsearch: coverage run runtests.py
@@ -21,23 +22,9 @@ basepython =
     py35: python3.5
 
 deps =
-    django-modelcluster>=1.1b1
-    django-taggit>=0.17.5
-    django-treebeard>=3.0,<5.0
     django-sendfile==0.3.6
-    djangorestframework==3.1.3
-    Pillow>=2.3.0
-    beautifulsoup4>=4.3.2
-    html5lib>=0.999,<1
-    Unidecode>=0.04.14
-    elasticsearch==1.1.0
-    mock==1.0.1
-    python-dateutil==2.2
-    pytz==2014.7
     Embedly
-    Willow>=0.3b4,<0.4
     jinja2==2.8
-    coverage
 
     dj18: Django>=1.8.1,<1.9
     dj19: Django>=1.9,<1.10