Update pipenv

bin/steps/pipenv

@@ -4,7 +4,7 @@ if [[ -f Pipfile ]]; then
     if [[ ! -f requirements.txt ]]; then
         puts-step "Generating 'requirements.txt' with pipenv"
 
-        pip install git+https://github.com/kennethreitz/pipenv.git#egg=pipenv &> /dev/null
+        pip install pipenv --upgrade &> /dev/null
         pipenv lock --requirements > requirements.txt 2> /dev/null
 
         pipstrip requirements.txt