Softlayer vpn on Yosemite not working

According to reports from @tomaszdurka and @kris-lab, upgrading to Yosemite made using SoftLayer's vpn unusable..

We could track down the problem to
* `/etc/ppp/ip-up` script having resetted permissions (`0644` instead of the needed `0755`)
* Softlayer's DNS being pushed with the ip address, but carrying an unreacheable address

Solution:
- Add a route to the dns servers
- Make sure permissions are correctly set (this should be taken care of by `install.sh`)