Add vendoring target to heck that vendoring is up-to-date

.travis.yml

@@ -17,6 +17,7 @@ jobs:
     - stage: primary
       env: TOXENV=docs
     - env: TOXENV=lint
+    - env: TOXENV=vendoring
     # Latest CPython
     - env: GROUP=1
       python: 2.7

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.4.0
 envlist =
-    docs, packaging, lint,
+    docs, packaging, lint, vendoring,
     py27, py35, py36, py37, py38, pypy, pypy3
 
 [helpers]
@@ -46,3 +46,15 @@ commands_pre =
 deps = pre-commit
 commands =
    pre-commit run [] --all-files --show-diff-on-failure
+
+[testenv:vendoring]
+skip_install = True
+commands_pre =
+deps =
+    invoke
+    requests
+whitelist_externals = git
+commands =
+    # Check that the vendoring is up-to-date
+    invoke vendoring.update
+    git diff --exit-code