circleci: switch to 2.1 config
circleci: switch to 2.1 config

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

siavashs/node_exporter,derekmarcotte/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,prometheus/node_exporter