travis-ci.org is being shut down, so switch to GitHub Actions.

It should be mostly equivalent, but I did drop functionality in a couple
cases:

- Publishing release binaries.  I don't think providing Linux binaries
  is useful, since people build their own anyway.  So I left this out.

- Build and testing on ppc64le.  GitHub Actions only natively supports
  x86.  I tried uraimo/run-on-arch-action, which uses Docker and QEMU
  user-mode emulation, but the fscrypt tests can't be run because
  QEMU user-mode emulation doesn't support all the needed system calls.

Enforce that all the cli-test scripts pass 'shellcheck'.

Add a framework for writing automated tests of the fscrypt command-line
tool.  See cli-tests/README.md for details.

As the Go community transitions to using the modules ecosystem,
we want to only support one way of managing dependencies.
So this change moves to only using Go modules for dependency management.

This means that our effective minimum Go version increases to Go 1.11.
To account for this, we also update:
  - the documentation
  - Makefile
  - CI scripts

These were found by a combination of manual review and a custom script
that checks for common errors.

Also removed an outdated sentence from the comment for setupBefore().

This fixes travis issues as well as moving us off of deprecated tooling

* Remove spelling mistakes in the repository
* Add travis script to check for typos.
* Add command to Makefile to check for typos.
* Fixes #71

Also clarifies some other minor points.

PR #85 failed to update the documentation. This is now fixed with some
additional cleanup.