When updating source always download the tags

[pmalek]

When upgrading to go1.15 on Mac (using git version 2.28.0) I've noticed that I keep getting Unrecognized Go version.

It appeared that latest master - ed2c95b - uses git pull without --tags which doesn't download tags on my machine, hence not finding the requested version (not sure how it worked before).

With this PR I suggest to add --tags to git pull there so that tags are always downloaded.

Maybe this will fix https://github.com/moovweb/gvm/issues/160