Update README-omni.txt for v1.5.
Update README-omni.txt for v1.5.

Add omni changes from CHANGES. Add the -V, --api-version command line
option to the list of supported options. Add the getusercred command
to the list of supported commands.

GENI-NSF/gram,ahelsing/geni-tools,plantigrade/geni-tools,tcmitchell/geni-tools,GENI-NSF/gram,ahelsing/geni-tools,GENI-NSF/gram,plantigrade/geni-tools,tcmitchell/geni-tools