fix return for old go version

I'm not a go expert, but I think that can fix this error: 
https://launchpadlibrarian.net/159564297/buildlog_ubuntu-raring-amd64.golang-uniri_0.1-0~15~ubuntu13.04.1_FAILEDTOBUILD.txt.gz
```
"src/github.com/dchest/uniuri/uniuri.go:48: function ends without a return statement"
```
according to this https://groups.google.com/forum/#!topic/golang-nuts/u66fQbfbzEo

Thank you :)

PS: JFYI, I created this: https://code.launchpad.net/~golang-uniuri/+archive/daily
a deb repository to use your package