Strip newlines from git describe output used for @PARENT_TAG@.

Not a python guy and did not run this, feel free to tell me what to change it to. I may try and figure out how to run this locally. The result I get for `<param name="versionformat">@PARENT_TAG@+git%cd.%h</param>` include a `\n` which does not seem intended.

The file name that is created:
```
obs-studio-0.6.4
+git20141109.ab7fa5b
```

which seems a bit scary.