Output pipenv version in steps (#662)

* Since pipenv is being pinned now, output pinned version in steps

Signed-off-by: Yury V. Zaytsev <yury.zaytsev@moneymeets.com>

* Update README.md example output to match new reality

Signed-off-by: Yury V. Zaytsev <yury.zaytsev@moneymeets.com>

* Update pipenv

README.md

@@ -25,7 +25,7 @@ Deploying a Python application couldn't be easier:
     -----> Python app detected
     -----> Installing python-3.6.4
     -----> Installing pip
-    -----> Installing requirements with latest pipenv…
+    -----> Installing requirements with Pipenv 11.7.1…
            ...
            Installing dependencies from Pipfile…
     -----> Discovering process types

bin/steps/pipenv

@@ -41,12 +41,14 @@ if [ ! "$SKIP_PIPENV_INSTALL" ]; then
             export PIP_EXTRA_INDEX_URL
         fi
 
+        export PIPENV_VERSION="11.8.2"
+
         # Install pipenv.
-        /app/.heroku/python/bin/pip install pipenv==11.7.1 --upgrade &> /dev/null
+        /app/.heroku/python/bin/pip install pipenv==$PIPENV_VERSION --upgrade &> /dev/null
 
         # Install the dependencies.
         if [[ ! -f Pipfile.lock ]]; then
-            puts-step "Installing dependencies with latest Pipenv…"
+            puts-step "Installing dependencies with Pipenv $PIPENV_VERSION…"
             /app/.heroku/python/bin/pipenv install --system --skip-lock 2>&1 | indent
         else
             pipenv-to-pip Pipfile.lock > requirements.txt
@@ -54,7 +56,7 @@ if [ ! "$SKIP_PIPENV_INSTALL" ]; then
             cp requirements.txt .heroku/python/requirements-declared.txt
             openssl dgst -sha256 Pipfile.lock > .heroku/python/Pipfile.lock.sha256
 
-            puts-step "Installing dependencies with latest Pipenv…"
+            puts-step "Installing dependencies with Pipenv $PIPENV_VERSION…"
             /app/.heroku/python/bin/pipenv install --system --deploy 2>&1 | indent
         fi