Migrate to CircleCI 2.0

[abesto]

As part of https://github.com/openzipkin/zipkin/issues/2102

Used https://github.com/openzipkin/zipkin-aws/pull/95 as a template; diff between the 1.0 circle.yml files:

# diff zipkin-{aws,azure}/circle.yml 
2c2
< # Copyright 2016-2017 The OpenZipkin Authors
---
> # Copyright 2017 The OpenZipkin Authors
16,17c16,17
<   environment:
<     JAVA_HOME: /usr/lib/jvm/zulu-9-amd64
---
>   java:
>     version: openjdk8
21,24d20
<     - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 0x219BD9C9
<     - sudo apt-add-repository -y "deb http://repos.azulsystems.com/ubuntu stable main"
<     - sudo apt-get -y update
<     - sudo apt-get -y install zulu-9

So essentially the same, except for the copyright year and that zipkin-azure is on JDK 8.

diff of 2.0 .circleci/config.yml after this PR:

# diff zipkin-{aws,azure}/.circleci/config.yml
2c2
< # Copyright 2016-2018 The OpenZipkin Authors
---
> # Copyright 2017-2018 The OpenZipkin Authors
19c19
<       - image: circleci/openjdk:9-jdk
---
>       - image: circleci/openjdk:8-jdk
51c51
<       - image: circleci/openjdk:9-jdk
---
>       - image: circleci/openjdk:8-jdk
66c66
<       - image: circleci/openjdk:9-jdk
---
>       - image: circleci/openjdk:8-jdk
81c81
<       - image: circleci/openjdk:9-jdk
---
>       - image: circleci/openjdk:8-jdk

[abesto]

Good thought on factoring out the docker image. I won't include that in this PR to preserve consistency across the CircleCI-built repos; sounds like a good improvement if we decide to stay with Circle / use it more long-term.

[codefromthecrypt]

good start. we will need to switch to 9 soon, but a straight port makes sense now.