Remove commit date from release zip

[aaemnnosttv]

Summary

This is a followup bug-fix to #1098

Addresses issue #1041

Relevant technical choices

• Removes commit date as part of the generated zip filename
  This is due to the way git-repo-info works by parsing commit data from .git/objects/xx/... rather than running git (see When run in docker, all but sha1 is null rwjblue/git-repo-info#46). However, these objects may not exist (such as after a fresh clone) causing the commiterDate to be null and thus the zip creation errors out when formatting the date (Cannot read property 'split' of null).
• Makes .git optional, so that the release zip can still be built without it

If .git is present (even after a fresh clone) the file format will be:
[email protected]

If .git is not present (i.e. zip download from GitHub) the file format will be:
google-site-kit.v1.2.3.zip

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).