Inconsistent CLI output format for GitCommitDate

[sychare]

Issue

Running nbgv get-version -f json and nbgv get-version -v GitCommitDate output different date formats and should be normalised to a machine readable format i.e. ISO 8601.

Actual outcome

> nbgv get-version -v GitCommitDate
26/05/2020 13:36:29 +01:00

> nbgv get-version -f json | sls 'gitcommitdate'
  "GitCommitDate": "2020-05-26T13:36:29+01:00",
    "NBGV_GitCommitDate": "26/05/2020 13:36:29 +01:00",

Expected outcome

> nbgv get-version -v GitCommitDate
2020-05-26T13:36:29+01:00

> nbgv get-version -f json | sls 'gitcommitdate'
  "GitCommitDate": "2020-05-26T13:36:29+01:00",
    "NBGV_GitCommitDate": "2020-05-26T13:36:29+01:00",

[AArnott]

Sounds good. Can you send a PR?