text: Title: Can't install development dependencies with documentation instructions
  Body: According to the [contribution guide](https://flask-restplus.readthedocs.io/en/latest/contributing.html), to install the development dependencies we should run `pip install -e .[dev]`. When I execute the command I get the following warning: 
  
  > flask-restplus 0.12.2.dev0 does not provide the extra 'dev'
  
  Some packages are installed, but not everything listed in requirements/develop.pip. Here is the result of pip freeze:
  ```bash
  $ pip freeze
  aniso8601==4.0.1
  Click==7.0
  Flask==1.0.2
  -e git+git@github.com:hygorxaraujo/flask-restplus.git@a8f35823fe40b2c7385632a2ad6b35b26467402c#egg=flask_restplus
  itsdangerous==1.1.0
  Jinja2==2.10
  jsonschema==2.6.0
  MarkupSafe==1.1.0
  pytz==2018.7
  six==1.11.0
  Werkzeug==0.14.1
  ```
  Steps to reproduce:
  1. Fork flask-restplus repository in GitHub
  2. Clone forked repository
  3. Create new branch `git checkout -b new-branch`
  4. Run `pip install -e .[dev]`
  
  Am I missing something or is the documentation outdated?
  Python: 3.6.7
  Flask-Restplus: 0.12.2.dev