Update dependency gatsby-plugin-google-gtag to v2

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
gatsby-plugin-google-gtag	dependencies	major	^2.1.1	source

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Release Notes

gatsbyjs/gatsby

v2.1.1

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v2.1.0

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v2.0.3

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v2.0.2

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v2.0.1

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v2.0.0

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