Add an image for git v2.16 and Chrome v63

Also adds a caching mechanism for storing the latest version in an S3 bucket.

Chrome v64 is still not quite ready for our testing purposes, so we may need to wait for v65. Until then, use a cache for Chrome V63 to support git 2.16.2.

• https://bugs.chromium.org/p/chromedriver/issues/detail?id=2239
• https://gitlab.com/gitlab-org/gitlab-ce/issues/43652.

Closes #17 (closed)