Add apt.txt to install jq during Binder build (#314)

jq is needed for parsing pyhf CLI output at the command line and is used in some Binder based examples in talks. If Binder finds an apt.txt file it will then proceed to install its contents with apt-get in the image it builds with repo2docker.

c.f. https://github.com/binder-examples/latex

Author: matthewfeickert

binder/apt.txt

@@ -0,0 +1 @@
+jq