Allowing modification check to run using "tox" (#3704)

#3337 and #3338 should ideally run when the user type tox. This allows them to catch the problem locally before they make a PR.

tox.ini

@@ -4,7 +4,7 @@
 
 [tox]
 skipsdist = true
-envlist = py{26,33,34,35},cover,lint
+envlist = modification,py{26,33,34,35},cover,lint
 
 # nosetest -v => more verbose output, allows to detect busy waiting
 # loops, especially on Travis
@@ -93,6 +93,12 @@ commands =
     pip install -e acme[dev] -e .[dev] -e certbot-nginx
     python certbot-compatibility-test/nginx/roundtrip.py certbot-compatibility-test/nginx/nginx-roundtrip-testdata
 
+# This is a duplication of the command line in testenv:le_auto to
+# allow users to run the modification check by running `tox`
+[testenv:modification]
+commands =
+    {toxinidir}/tests/modification-check.sh
+
 [testenv:le_auto]
 # At the moment, this tests under Python 2.7 only, as only that version is
 # readily available on the Trusty Docker image.