travis-before-script.sh: add ppa repository

Ubuntu 12.04 has not libgstreamer1.0-dev package in official repository.

```
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=12.04
DISTRIB_CODENAME=precise
DISTRIB_DESCRIPTION="Ubuntu 12.04.4 LTS"

$ LANG=C apt-cache show -n libgstreamer1.0-dev
N: Unable to locate package libgstreamer1.0-dev
N: Couldn't find any package by regex 'libgstreamer1.0-dev'
E: No packages found
```

It needs add ppa repository which is ppa:gstreamer-developers/ppa.