thumbor: Add dependencies and virtualenv setup script.

In this commit we add new dependencies needed for running thumbor.
Also we add the script for creating the virtual environment ready
for thumbor.
Note: Thumbor will use python2 and thus have different virtualenv
dedicated to it.
Credits to @TigorC and @joshland as well for there work on this.

scripts/lib/setup_venv.py

@@ -37,6 +37,14 @@ VENV_DEPENDENCIES = [
     "libpq-dev",            # Needed by psycopg2
 ]
 
+THUMBOR_VENV_DEPENDENCIES = [
+    "libcurl4-openssl-dev",
+    "libjpeg-dev",
+    "libjasper-dev",
+    "libpng-dev",
+    "gifsicle",
+]
+
 def install_venv_deps(requirements_file):
     # type: (str) -> None
     pip_requirements = os.path.join(ZULIP_PATH, "requirements", "pip.txt")