Make start.sh cope when gnome-terminal is missing, e.g. on Mac OSX

[daj]

Relates to: #29

gnome-terminal doesn't exist on Mac OSX, so we need to cope without it. Calling vncviewer directly opens it in a new window, so that works fine (even though for me it currently shows an empty environment in RVis).

TESTING

Before fix:

$ ./start.sh 
Creating minio ... done
Creating rl_coach ... done
Creating robomaker ... done
waiting for containers to start up...
attempting to pull up sagemaker logs...
./start.sh: line 12: gnome-terminal: command not found
attempting to open vnc viewer...
./start.sh: line 15: gnome-terminal: command not found

After fix, with both errors happening:

$ ./start.sh
minio is up-to-date
Starting rl_coach ... done
robomaker is up-to-date
waiting for containers to start up...
Error: skip showing sagemaker logs because gnome-terminal is not installed.  This is normal if you are on a different OS to Ubuntu.
Error: vncviewer is not present on the PATH.  Make sure you install it and add it to the PATH.

After fix, with vncviewer added to the PATH (using sudo vi /etc/paths):

$ ./start.sh 
minio is up-to-date
Starting rl_coach ... done
robomaker is up-to-date
waiting for containers to start up...
Error: skip showing sagemaker logs because gnome-terminal is not installed.  This is normal if you are on a different OS to Ubuntu.
attempting to open vnc viewer...