Only report version check when it can be

[stoned]

... otherwise you could get something like the following (here sops is run without network connection available):

$ ./sops --version
sops 3.7.3
[warning] failed to retrieve latest version from upstream: Get "https://raw.githubusercontent.com/mozilla/sops/master/version/version.go": dial tcp: lookup raw.githubusercontent.com: Temporary failure in name resolution

[warning] failed to compare current version with latest: Version string empty
 (latest)
$