fix: fix linting errors in the samples (#100)
fix: fix linting errors in the samples (#100)

`eslint` was reporting the following error:
```
error  Async functions are not supported until Node.js 7.6.0. The configured version range is '>=6.0.0'  node/no-unsupported-features/es-syntax
```
This happened because `eslint` has a new `node/no-unsupported-features/es-syntax`
that by default reports an error.

The error occurs because the samples use async/await, async/await is not available
until Node >= 7.6.0, and the samples `package.json` specifies only Node >= 6.0 is required.

To fix the linting errors, the `node/no-unsupported-features/es-syntax` was configured to
only issue a warning.

This was done, instead of updating the `package.json` to specify that Node >= 7.6.0 is
required,   since the library `@google-cloud/logging-bunyan` library only requires
Node >= 6.0.0.  It is only if you want to use async/await that Node >= 7.6.0 is required.
googleapis/nodejs-logging-bunyan,googleapis/nodejs-logging-bunyan,googleapis/nodejs-logging-bunyan