Output the installed version of pip in the build log (#1007)

Since:
* "explicit is better than implicit"
* we'll soon be upgrading pip, and debugging breakage caused by upgrades
  will be easier if versions are visible in the build log

Closes #939.

bin/steps/python

@@ -150,7 +150,7 @@ fi
 # If a new Python has been installed or Pip isn't up to date:
 if [ "$FRESH_PYTHON" ] || [[ ! $(pip --version) == *$PIP_UPDATE* ]]; then
 
-  puts-step "Installing pip"
+  puts-step "Installing pip ${PIP_UPDATE}"
 
   # Remove old installations.
   rm -fr /app/.heroku/python/lib/python*/site-packages/pip-*