Remove notebook dependencies from requirements-dev.txt

[garrison]

Summary

This removes the notebook dependencies from requirements-dev.txt, as otherwise they are specified in two places. This PR supersedes #37.

Details and comments

[garrison]

I've experienced the following error when running tox -elint on this branch on one machine, but not on my primary workstation:

************* Module tests.test_backends
tests/test_backends.py:20:0: E0401: Unable to import 'qiskit.providers.ibmq' (import-error)
tests/test_backends.py:20:0: E0611: No name 'ibmq' in module 'qiskit.providers' (no-name-in-module)

------------------------------------------------------------------
Your code has been rated at 9.91/10 (previous run: 9.91/10, +0.00)

ERROR: InvocationError for command /tmp/prototype-qrao/.tox/lint/bin/pylint -rn --py-version=3.6 --disable=fixme,invalid-name,missing-function-docstring,protected-access,duplicate-code qrao tests (exited with code 14)
___________________________________ summary ____________________________________
ERROR:   lint: commands failed

All other tox environments work, even on the machine where this one fails.

I think I'll merge this soon anyway, as it will presumably be resolved once #15 is merged, in case it ever crops up again.