This switches Zap to using Go modules.

To keep the list of direct dependencies of Zap minimal, benchmarks are
placed in their own Go module as per [Multi-Module Repositories][1].

This change deletes the glide.lock but keeps the glide.yaml around for
legacy ecosystems.

[1]: https://github.com/golang/go/wiki/Modules#faqs--multi-module-repositories

With Go modules, the location of global_test on disk does not have to be
in `$GOPATH/src/go.uber.org/zap`, or even in a directory named `zap`.
This simplifies the assertion to verify just the file name.

This fixes TestStacktraceFiltersVendorZap to work with Go modules.

For context, the test previously did the following:

- Set up a temporary GOPATH
- Copy stacktrace_ext_test.go into it
- Symlink Zap and relevant dependencies into a vendor/ directory in the
  temporary GOPATH
- Run specific tests from stacktrace_ext_test.go with Zap inside the
  vendor/ directory

We're no longer using GOPATH or vendor/ directories, but this behavior
of Zap when *it* is inside a vendor/ directory should be retained.

To that end, this changes TestStacktraceFiltersVendorZap to continue to
run with the following changes:

- Use `go mod download -json` to retrieve the locations of all of Zap's
  dependencies on-disk, and symlink them into the temporary GOPATH's
  vendor/.
- Explicitly set `GO111MODULE=off` before running `go test` because we
  may be running in an environment where `GO111MODULE=on` is set.

The benchmarks package has been placed inside its own module so that its
dependencies are not considered Zap's dependencies.

This changes the README update script to run the benchmark inside the
benchmark module's directory since we can no longer invoke it from Zap's
top-level directory.

This switches the Makefile and travis setup to rely on Go modules and
`./...` instead of `glide nv`.

Note that a tradeoff of making `benchmarks/` its own module is that
`./...` in the Zap directory will no longer include the `benchmarks/`
directory. So where necessary, we need to run commands separately in
that directory as well, which complicates the build script ever so
slightly.

Rather than hard-coding the Go version we lint against in the Makefile,
this switches to managing that in the .travis.yml -- the same place we
manage the versions we're testing against.