Update min version to build from source

[deefour]

1. What does this change do, exactly?

Updates the minimum version of Go required to build from source. 1.8 fails since #2009 from the introduction of sync.Map

2. Please link to the relevant issues.

#2009

3. Which documentation changes (if any) need to be made because of this PR?

This is the doc change.

4. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code
• I am willing to help maintain this change if there are issues with it later

[CLAassistant]

All committers have signed the CLA.

[mholt]

Thanks!