Using a pip cache to speed up deploys

tox.ini

@@ -10,9 +10,9 @@ setenv =
 passenv = *
 usedevelop = true
 commands =
-    pip install -U pip
-    pip install -U setuptools
-    pip install -Ue {toxinidir}[testing]
+    pip install -U pip --cache-dir /var/cache/pip
+    pip install -U setuptools --cache-dir /var/cache/pip
+    pip install -Ue {toxinidir}[testing] --cache-dir /var/cache/pip
     {toxinidir}/src/memopol/bin/install_client_deps.sh
     cp {toxinidir}/src/memopol/tests/local_settings.py {toxinidir}/src/memopol/local_settings.py
     py.test src/
@@ -40,9 +40,9 @@ changedir = {toxinidir}
 basepython = python2.7
 passenv = *
 commands =
-    pip install -U pip
-    pip install -U setuptools
-    pip install -Ue {toxinidir}[testing]
+    pip install -U pip --cache-dir /var/cache/pip
+    pip install -U setuptools --cache-dir /var/cache/pip
+    pip install -Ue {toxinidir}[testing] --cache-dir /var/cache/pip
     flake8 src --exclude '*/migrations,docs,static' --ignore E128
 
 [testenv:docs]