Make launch script more resilient

Fixed quoting so that the script can launch Tribler when there are
spaces in the path to the script. To see the problem without the patch,
try this scenario:

git clone --recursive https://github.com/Tribler/tribler.git "tribler repo"
cd "tribler repo"
./tribler.sh

Changed to sh instead of Bash since no Bash features were being used and
sh is more portable.
Oh man, you are right, the command gets translated to its full path even when when called from the PATH!

Thanks!