How can I modify Info.plist under EI Capitan 10.11.4 or Later
The culprit (as suggested by klanomath) is likely System Integrity Protection, that protects (except others) also the /usr directory. To switch it off


Switch off your Mac
Boot into recovery (when you press power button, you should be already holding cmdR keys, hold them as long as the display is clear white)
In the recovery system, go in the menubar to Utilities > Terminal 
Terminal window will pop-up. Enter: 

csrutil disable
Confirm with enter
Close Terminal, restart the computer through Apple logo in top left corner


From here on, the SIP is disabled and you can edit your file. It should be noted, that after you are done with your modifications, you should definitely re-enable the SIP. The method is same as above, the only difference is that you will type into Terminal:

csrutil enable