don't upgrade pip, use default version

[neuhausler]

upgrade pulled newest 19.0 version, which out of the box, without re-configuration, runs in to an error with --no-cache-dir:

Exception:
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/site-packages/pip/_internal/cli/base_command.py", line 176, in main
status = self.run(options, args)
File "/opt/conda/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 346, in run
session=session, autobuilding=True
File "/opt/conda/lib/python3.6/site-packages/pip/_internal/wheel.py", line 848, in build
assert building_is_possible
AssertionError

[ptwobrussell]

@neuhausler - Thanks for this PR! I've asked @mikhailklassen to do the review on it. (He may have previously overlooked it since it was submitted but not assigned.)

[mikhailklassen]

Thanks for the PR! I've merged it in for now. Was the later version of pip giving you problems?

I'm still debating keeping in the code for downloading the NLTK package. Even if that requirement has been satisfied in the Dockerfile, not all users may be running the code from a Docker container, and it's illustrative nevertheless of what NLTK is using for the example.