Force git to return something if tags aren't found

If there are no tags in the current repository, this command will fail leaving the OSQUERY_BUILD_VERSION empty, and therefore breaking the package building process (and presumably other things too) due to the empty version flag.  By adding the flag --always, this forces git to fallback to a commit id instead of returning nothing.

Before:
```
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ git describe --tags HEAD
fatal: No names found, cannot describe anything.
```
After
```
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ git describe --tags HEAD --always 
3716418
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ git tag 0.1.1-working-modified
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ git describe --tags HEAD --always
0.1.1-working-modified
```