use circleci image which supports ruby

[punchdrunker]

Issue

• close #N.A.

Overview (Required)

• use official docker image

as I remember that circleci image didn't support ruby, but now they apparently support ruby in android image.

Links

• https://github.com/CircleCI-Public/circleci-dockerfiles/blob/master/android/images/api-27-alpha/Dockerfile

Screenshot

Before	After

[punchdrunker]

they have supported ruby from the beginning. my memory was wrong :angel: https://github.com/CircleCI-Public/circleci-dockerfiles/commit/714e74949667ad4b78791bc28869657733d347e4#diff-d00fc552f4ee3d1f578fcef860f25cd4

[takahirom]

Thanks!