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.

Now that Travis CI supports a version of Ubuntu that has a kernel that
supports v2 encryption policies, upgrade to it and enable the cli tests.

Don't let people check in code that breaks 32-bit builds.

Update https://github.com/google/fscrypt/issues/233

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

There's no longer a need to override the Ubuntu version that the
Travis CI builds install e2fsprogs from, since we now use
"dist: bionic", and e2fsprogs in Bionic supports encryption.

* Release version v0.2.6

Fixes #195

Also, update the encrypted API key.
My person access token had expired, this one should work now.

Signed-off-by: Joe Richey <joerichey@google.com>

Ensure that the environmental variable GO111MODULE is set to "on" when
running 'go get'.  This fixes a CI failure with Go 1.11 and 1.12.

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

Fixes CI issues