title: Tox WARNING:test command found but not installed in testenv
body: <p>I am using tox for my project.</p>
  <p>Here is my <code>tox.ini</code> file:</p>
  <pre><code>[tox]
  envlist =
      py27,
      lint,
      coverage
  
  skipsdist = True
  
  [testenv:py27]
  deps = -rrequirements.txt
  commands = python -m unittest discover -s ./tests
  
  [testenv:coverage]
  commands =
      coverage run --source=tests -m unittest discover -s tests/
      coverage html
      coverage report
  
  
  [testenv:lint]
  commands = pylint ./foo
  </code></pre>
  <p>whenever I run tox, everything is getting executed which basically is linting, coverage.</p>
  <p>but Tox is displaying warning for everything.</p>
  <pre><code>WARNING:test command found but not installed in testenv
  Maybe you forgot to specify a dependency? See also the whitelist_externals envconfig setting.
  </code></pre>
  <p>Everything succeeds, but it is still displaying warning and errors. Can anyone tell me to what I am doing wrong?</p>
  <p>My <code>requirements.txt</code> file is this:</p>
  <pre><code>requests==2.18.4
  JsonForm==0.0.2
  jsonify==0.5
  jsonschema==2.6.0
  JsonSir==0.0.2
  python-dateutil==1.5
  DateTime==4.2
  urllib3==1.22
  contextlib2==0.5.5
  mock==2.0.0
  patch==1.16
  </code></pre>