Add requirements.txt

Generated with `pip freeze`. This is so I can pin the versions of
packages that I've tested with as well as to allow Travis to test
whether the project runs on various Pythons and to let GitHub in-
form me of vulnerabilities in some libraries.

README.md

@@ -8,7 +8,7 @@ Create a virtual environment and run it:
 
     $ virtualenv -p /usr/bin/python3.6 venv
     $ . venv/bin/activate
-    $ pip install falcon gunicorn SolrClient psycopg2-binary
+    $ pip install -r requirements.txt
     $ gunicorn app:api
 
 ## Todo