updates dependencies to more accurately reflect the repo

[bshaffer]

Exchanges ext-grpc for google/cloud-logging

[bshaffer]

Some context:

1. Stackdriver Logging is capable of reporting errors to Stackdriver Error Reporting using the jsonPayload field
2. The Stackdriver Logging API parses this field and when certain attributes exist (such as reportLocation.functionName, serviceContext.module, and serviceContext.version), the log is reported as an error.
3. We have a handwritten Logging client and a gRPC-only Error reporting client. Additionally, @tmatsuo has added some support for Bootstrapping the PHP Exception/Error/Shutdown handlers and using the Logging client. All of these work specifically with the Batch Runner and the PHP runtime.

Moving forward with the current state of things, I've made samples (and documentation) to use the Logging client for error reporting and use some of the Bootstrapping for registering the error handlers (see this PR). However, error reporting is something most users will just want to have work with as little effort as possible, and we are not there yet. Hopefully we'll be there before this library goes beta.

Some things I want to have by then:

1. a one-liner for registering the error handler
2. a one-liner for reporting an error
3. samples which use google/cloud-error-reporting instead of google/cloud
4. documentation for integrating the error handler into wordpress, laravel, symfony, and possibly others.
5. documentation for integrating into GCE and GKE

This PR is an effort to move in that direction, so that users don't need gRPC in order to log exceptions.

[bshaffer]

We missed 0.40 on this, but I would love to get this merged for the next release.

[jdpedrie]

cc @tmatsuo