This PR contains a number of smaller changes aimed at broadly improving Gridsync's CI/build process in various ways. Among them:

• Update Travis-CI config to cache pip and pyenv-related directories, resulting in a drastic (~50%) reduction of build/CI wait times
• Update Travis-CI config to run the test suite under python3.7 (in addition to 3.5 and 3.6)
• Update Travis-CI config to run the test suite on newer versions of macOS (10.12, 10.13, 10.14) and Ubuntu (16.04), while dropping Ubuntu 14.04 (which becomes "EOL" this month)
• Decouple the make_archive and sha256sum functions from the PyInstaller .spec files, allowing them to be run separately (e.g., as an individual buildbot-worker step)
• Update the build script/Makefile(s) to re-use already-existing/previously-created PyInstaller virtualenvs and build artifacts, preventing needless re-building
• Pin versions and provide/check/require sha256 hashes for all dependencies -- including those used by tox
• Add a pyinstaller toxenv for building the application in a more "standardized" way (as opposed to creating virtualenvs via a shell script wrapped inside a make target)
• Remove old make targets, build scripts, and setup.py options that are no longer needed or in use (e.g., those relating to py2app, building Qt from source, etc.)