thumbor: Fix missing virtualenv-clone package in dependencies.

This package is important in order to avoid scary-looking errors
whenever we upgrade the dependencies in thumbor.txt (where
virtualenv-clone isn't installed in the venv, and then gets installed
by the code we just added a TODO comment to.

requirements/thumbor.in

@@ -13,3 +13,6 @@ backports-abc==0.5
 backports.ssl-match-hostname==3.5.0.1
 certifi==2018.1.18
 singledispatch==3.4.0.3
+
+# Needed for cloning virtual environments
+virtualenv-clone==0.3.0

requirements/thumbor.txt

@@ -37,4 +37,5 @@ thumbor==6.4.2
 tornado-botocore==1.3.2   # via tc-aws
 tornado==4.5.3            # via thumbor, tornado-botocore
 typing==3.6.4
+virtualenv-clone==0.3.0
 webcolors==1.8.1          # via thumbor

scripts/lib/setup_venv.py

@@ -157,6 +157,9 @@ def try_to_copy_venv(venv_path, new_packages):
                                                     source=source_venv_path,
                                                     target=venv_path).split()
         try:
+            # TODO: We can probably remove this in a few months, now
+            # that we can expect that virtualenv-clone is present in
+            # all of our recent virtualenvs.
             run(cmd)
         except Exception:
             # Virtualenv-clone is not installed. Install it and try running