Problem with pluginOptions.gtagId

[carminati-marco]

In /src/gatsby-ssr.js, there is a check to pluginOptions.gtagId, a vars which is never used (probably must be replaced with pluginOptions.trackingId or viceversa).

Note: in documentation we don't have gtagId but trackingId; it must be updated in order this library must be used.

[jakemmarsh]

I've opened a PR for this here: #4