Add old dependency test

tox.ini

@@ -6,7 +6,7 @@
 # acme and letsencrypt are not yet on pypi, so when Tox invokes
 # "install *.zip", it will not find deps
 skipsdist = true
-envlist = py{26,27,33,34,35},cover,lint
+envlist = py{26,27,33,34,35},py{26,27}-oldest,cover,lint
 
 # nosetest -v => more verbose output, allows to detect busy waiting
 # loops, especially on Travis
@@ -31,6 +31,15 @@ setenv =
     PYTHONHASHSEED = 0
 # https://testrun.org/tox/latest/example/basic.html#special-handling-of-pythonhas
 
+
+deps =
+    py{26,27}-oldest: cryptography==0.8
+    py{26,27}-oldest: configargparse==0.10.0
+    py{26,27}-oldest: psutil==2.1.0
+    py{26,27}-oldest: PyOpenSSL==0.13
+    py{26,27}-oldest: pyparsing==1.5.5
+    py{26,27}-oldest: python2-pythondialog==3.2.2rc1
+
 [testenv:py33]
 commands =
     pip install -e acme[testing]