Update sync_gateway_service_install.sh

I couldn't get this script to run on a default install of Ubuntu 14.04.1 LTS. The error I received was:
```
./sync_gateway_service_install.sh: 130: [: missing ]
```
Making this small tweak fixed things.
Merge pull request #534 from vfleurima/patch-1

Update sync_gateway_service_install.sh