Support for old Git versions

The command used to get the previous tag on a branch is not compatible with old versions of Git:

```
git log HEAD --pretty=format:%d
```

The code looks for `(tag: ***)` patterns and those are not present yet in Git 1.7.12.4 on CentOS 5.

Using:

```
git log HEAD --pretty=oneline --decorate
```

instead.
