Avoid using require to read package version

[dzbarsky]

Using this library with Webpack and yarn PnP results in the following warning:

../../../.yarn/__virtual__/@google-cloud-logging-winston-virtual-bfe17c7235/0/cache/@google-cloud-logging-winston-npm-5.1.5-9bf0067665-24b9d19918.zip/node_modules/@google-cloud/logging-winston/build/src/common.js
Critical dependency: the request of a dependency is an expression

Import trace for requested module:
../../../.yarn/__virtual__/@google-cloud-logging-winston-virtual-bfe17c7235/0/cache/@google-cloud-logging-winston-npm-5.1.5-9bf0067665-24b9d19918.zip/node_modules/@google-cloud/logging-winston/build/src/common.js
../../../.yarn/__virtual__/@google-cloud-logging-winston-virtual-bfe17c7235/0/cache/@google-cloud-logging-winston-npm-5.1.5-9bf0067665-24b9d19918.zip/node_modules/@google-cloud/logging-winston/build/src/index.js

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Fixes #762

[dzbarsky]

Sounds great, thanks!