Pin to pip 9.0.2 for pipenv users only

This addresses an issue raised by @CaseyFeist during code review:

  Updating pip for pipenv users or requiring them to update without a
  heads up won't be a good experience (our version is old enough that
  they'll need to uninstall and reinstall pipenv locally to successfully
  update). If you can refactor this to stay pinned to current version for
  pipenv users only, I should be able to accept this (and the related
  project updates).

  heroku/heroku-buildpack-python#833 (comment)

bin/compile

@@ -77,6 +77,11 @@ for file in "$BUILD_DIR/runtime.txt" "$CACHE_DIR/.heroku/python-version" ; do
     esac
 done
 
+if [[ -f "$BUILD_DIR/Pipfile" ]]; then
+    # Do not force pipenv users to re-install pipenv locally.
+    PIP_UPDATE="9.0.2"
+fi
+
 export DEFAULT_PYTHON_STACK PIP_UPDATE
 export PY37 PY36 PY35 PY27 PY34