[Doc] Target protobuf 2.6 in OS X build notes.
[Doc] Target protobuf 2.6 in OS X build notes.

Homebrew now installs Protobuf version 3 by default, which doesn't currently compile. Install Protobuf 2.6.x from the versions tap instead.

Github-Pull: #8754
Rebased-From: b16a7f609fdb2efa9a8a1b82f09748b95b5ae4f6

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