Use pipenv for managing development dependencies

* Add `Pipfile[.lock]`
* Update `CONTRUBUTING.md` instructions
* Drop the `test-requirements.txt` file (no longer needed)

CONTRIBUTING.md

@@ -37,17 +37,16 @@ brew cask install vagrant
 Now you can setup your virtual environment for testing:
 
 ```bash
-virtualenv --python=$(which python2.7) .venv
-source .venv/bin/activate
-pip install --no-deps -r tests/test-requirements.txt
+pip install -U pipenv
+pipenv install --dev
 ```
 
 Run the full lifecycle test on a given `<PLATFORM>`. Allowed values for
 `<PLATFORM>` are `mojave`, `highsierra`, `sierra`, `elcapitan`, and `yosemite`.
 
 ```bash
 source molecule/<PLATFORM>.sh
-molecule test
+pipenv run molecule test
 ```
 
 Unset the `MOLECULE_` environment variables:
@@ -56,6 +55,12 @@ Unset the `MOLECULE_` environment variables:
 source molecule/unset.sh
 ```
 
+Check for security vulnerabilities in the dependencies:
+
+```bash
+pipenv check
+```
+
 What if I just have a question?
 -------------------------------
 

Pipfile

@@ -0,0 +1,16 @@
+[[source]]
+url = "https://pypi.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[dev-packages]
+ansible = "<2.8.0"
+molecule = {extras = ["vagrant"],version = "<3.0.0"}
+testinfra = "<2.0.0"
+ansible-lint = "<4.0.0"
+yamllint = "<2.0.0"
+requests = "<3.0.0"
+python-vagrant = "*"
+
+[requires]
+python_version = "2.7"