Fix time format RFC3339 in version #283

[u5surf]

Fixes #283
I added time.Format(time.RFC3339) in version

[CLAassistant]

All committers have signed the CLA.

[masci]

@u5surf thanks for the PR, my apologies but you got caught right in the middle of an invasive merge, can you rebase on master and solve the conflicts? 🙇‍♂️

[mastrolinux]

The PR now perfectly fixes the standard output format, it does not solve yet the --format json case.

$ ./arduino-cli version 
arduino-cli Version: 0.3.7-alpha.preview Commit:  BuildDate: 2019-07-17T15:24:18Z
$ ./arduino-cli version --format json
{
  "Application": "arduino-cli",
  "VersionString": "0.3.7-alpha.preview",
  "Commit": "",
  "BuildDate": "2019-07-17T15:27:41.7405288Z"
}

Do you like to update the PR to completely fix #283 ?
Thanks for your help @u5surf !

[masci]

We could add a ToJSON() method to the VersionInfo struct so we keep the format definition in one place

[u5surf]

@mastrolinux I updated the PR to fix the both of format.
@masci Thx reviewing. After all, I implemented the new type rfc3339Time which has the string formatter and JsonMarshal function.

[masci]

Love it! Waiting for @mastrolinux to confirm the format is what we want but code LGTM