Enable minimal CircleCI setup

[shwang]

Configures a minimal CircleCI setup. I uploaded to DockerHub
a minimal Docker image bundling the Ubuntu and Java dependencies
that are required to pip install -e . MineRL. CircleCI pulls
this Docker image and runs pytest on top of it.

Because the CircleCI setup is not complete, we should run both BuildKite and
CircleCI for "hybrid" testing until we complete address the issues below:

This PR adds conditional skips to some data pytests when MINERL_DATA_ROOT is
not set. This is because the minimal CircleCI set up doesn't download
and cache any MineRL datasets right now. I'm leaving that for future
work.

Other future work:

• Pytests are not parallelized yet.
• Maybe transfer ownership of springulum/minerl-circleci-base Docker image
  to a shared MineRL account. (But I don't expect that we actually need to
  update this image for a long time, since its just a bunch of Java / Ubuntu packages.)

[shwang]

oof, it's failing on CircleCI's server (during MineRL Gradle build), but not on my local CircleCi runs. Maybe this is because we don't have enough memory in the free Runners offered to open source projects.

[shwang]

ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 FAILURE: Build failed with an exception.
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 * What went wrong:
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 Gradle build daemon disappeared unexpectedly (it may have been killed or may have crashed)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 * Try:
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 Run with --info or --debug option to get more log output. Run with --scan to get full insights.
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 * Exception is:
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 org.gradle.launcher.daemon.client.DaemonDisappearedException: Gradle build daemon disappeared unexpectedly (it may have been killed or may have crashed)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at org.gradle.launcher.daemon.client.DaemonClient.handleDaemonDisappearance(DaemonClient.java:242)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at org.gradle.launcher.daemon.client.DaemonClient.monitorBuild(DaemonClient.java:218)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at org.gradle.launcher.daemon.client.DaemonClient.executeBuild(DaemonClient.java:179)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at org.gradle.launcher.daemon.client.DaemonClient.execute(DaemonClient.java:142)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at 
[...]
org.gradle.wrapper.BootstrapMainStarter.start(BootstrapMainStarter.java:30)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at org.gradle.wrapper.WrapperExecutor.execute(WrapperExecutor.java:127)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 	at org.gradle.wrapper.GradleWrapperMain.main(GradleWrapperMain.java:61)
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 
ERROR    minerl.env.malmo.instance.41f52f:malmo.py:526 * Get more help at https://help.gradle.org
DEBUG    minerl.env._multiagent:_multiagent.py:660 Closing MineRL env...

[shwang]

OK, we are ready to push this -- we've got CircleCI running on tests that don't launch Minecraft, and it takes about 2.5 minutes to finish those tests. We can try launching Minecraft when we figure out memory requirements later.

[brandonhoughton]

WOO! This is great =)