fix(ts): explicit usage of Duplex

[bchhun]

because otherwise, we get syntax error messages when we try to import bigquery in another project

Fixes #299 (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

The npm run lint command has errors but not explicitly in the part I've modified:

❯ npm run lint

> @google-cloud/bigquery@2.0.3 lint /Users/bchhun/projects/nodejs-bigquery
> gts check && eslint samples/


/Users/bchhun/projects/nodejs-bigquery/samples/datasets.js
  21:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  41:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  64:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require

/Users/bchhun/projects/nodejs-bigquery/samples/queries.js
  22:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  66:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  97:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require

/Users/bchhun/projects/nodejs-bigquery/samples/quickstart.js
  20:28  error  "@google-cloud/bigquery" is not found  node/no-missing-require

/Users/bchhun/projects/nodejs-bigquery/samples/system-test/datasets.test.js
  18:28  error  "@google-cloud/bigquery" is not found  node/no-missing-require

/Users/bchhun/projects/nodejs-bigquery/samples/system-test/tables.test.js
  23:28  error  "@google-cloud/bigquery" is not found  node/no-missing-require

/Users/bchhun/projects/nodejs-bigquery/samples/tables.js
   21:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
   49:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
   74:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
   96:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  128:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  161:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  193:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  242:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  291:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  358:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  417:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  472:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  526:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  588:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  649:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  704:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  765:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require
  805:30  error  "@google-cloud/bigquery" is not found  node/no-missing-require

✖ 27 problems (27 errors, 0 warnings)

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @google-cloud/bigquery@2.0.3 lint: `gts check && eslint samples/`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the @google-cloud/bigquery@2.0.3 lint script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/bchhun/.npm/_logs/2018-12-10T22_40_17_760Z-debug.log

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[bchhun]

I signed it!

[googlebot]

CLAs look good, thanks!

[JustinBeckwith]

Thank you for the fix!