Updated CHANGELOG.md for 1.5.1 release date

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• InfluxQL Spec updated
• Provide example syntax
• Update man page when modifying a command
• Config changes: update sample config (etc/config.sample.toml), server NewDemoConfig method, and Diagnostics methods reporting config settings, if necessary
• InfluxData Documentation: issue filed or pull request submitted <link to issue or pull request>