Fix pip uninstall by moving it prior to pip install (#413)

The pip-uninstall step stopped working when it was moved to after
the pip-install step in f27a84e.

This regression was temporarily fixed by part of #397, however that
PR was reverted in #404.

Adds a test to hopefully catch any future regressions :-)

Fixes #393.

test/run

@@ -62,6 +62,16 @@ testPython3() {
   assertCapturedSuccess
 }
 
+testSmartRequirements() {
+  local cache_dir="$(mktmpdir)"
+  compile "requirements-standard" "$cache_dir"
+  assertFile "requests" ".heroku/python/requirements-declared.txt"
+  assertCapturedSuccess
+  compile "psycopg2" "$cache_dir"
+  assertCaptured "Uninstalling requests"
+  assertFile "psycopg2" ".heroku/python/requirements-declared.txt"
+  assertCapturedSuccess
+}