Rework requirements for pyup.io

[alimanfoo]

This PR reworks the requirements to allow for automated management via the PyUP bot. Changes include:

• All indirect requirements have been removed from requirements files.
• Only direct requirements that are imported in the main codebase are pinned and will be tracked by the PyUP bot. These are given in requirements_dev* files.
• Requirements needed for running tests are removed to requirements_test.txt and are unpinned to avoid too much noise from PyUP.
• pip freeze has been added to CI scripts to ensure we can debug exactly what packages were installed into the environment for any given CI run.
• I've also tentatively removed 32-bit windows runs from appveyor to reduce CI overhead. Has been discussed over on numcodecs (Support timeline for Windows 32-bit numcodecs#97). Happy to revert this if needs more discussion.

TODO:

• Changes documented in docs/release.rst
• Docs build locally (e.g., run tox -e docs)
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[alimanfoo]

Tests are currently failing on PY37 because numcodecs is not building C extensions, I believe this is because I removed cython from the test environments and so an attempt is being made to build from generate C sources, but in current numcodecs release those are not compatible with PY37. This has been resolved upstream in numcodecs (zarr-developers/numcodecs#92) but that has not been released yet. A workaround is to ensure cython is ensured in the zarr CI environments, so numcodecs gets cythonized during install.

[alimanfoo]

OK, CI is green. @zarr-developers/core-devs any objections?

[jhamman]

LGTM, nice to see the simplified dev environment.

[alimanfoo]

Hm, can't figure out why coveralls has dropped since last commit. Looks like PY2 branches are not getting covered, but the PY2 CI run on travis completed fine.

[alimanfoo]

Maybe coveralls bug? Only affects storage.py, lines are marked as "uncov" yet have "1x" coverage.

[alimanfoo]

Rerunning Travis PY27 to see if coveralls gets fixed.

[alimanfoo]

Ah, sorry, there is one line uncovered, looks like it snuck in from #268. Will merge this and deal with that separately.