Fix python install utility when not using Heroku

Don't try to install `requirements.txt` which doesn't exist unless using Heroku.

{{cookiecutter.project_slug}}/utility/install_python_dependencies.sh

@@ -45,6 +45,8 @@ else
 
     pip install -r requirements/local.txt
     pip install -r requirements/test.txt
+    {% if cookiecutter.use_heroku == "y" -%}
     pip install -r requirements.txt
+    {%- endif %}
 fi