put a function_exists check in place to protect installs using PHP < 5.3
resolves https://github.com/twilio/OpenVBX/issues/236
incrementing patch level to reflect bug fix;
Merge branch 'hotfix/json_last_error' into develop
Fix for timing not allowing open on Sundays.
Interesting off by one error, 'w' is Sun-Sat:0-6, 'N' is Mon-Sun:1-7.
Credit goes to tjlytle: https://github.com/tjlytle/OpenVBX/commit/e6b7d2156ad23339215661b85310270b893be051