Fix Twig deprecation warnings

[mpdude]

Using an instance of "Twig_Function_Method" for function "calendr_year" is deprecated since version 1.21. Use Twig_SimpleFunction instead.
Using an instance of "Twig_Function_Method" for function "calendr_month" is deprecated since version 1.21. Use Twig_SimpleFunction instead.
Using an instance of "Twig_Function_Method" for function "calendr_week" is deprecated since version 1.21. Use Twig_SimpleFunction instead.
Using an instance of "Twig_Function_Method" for function "calendr_day" is deprecated since version 1.21. Use Twig_SimpleFunction instead.
Using an instance of "Twig_Function_Method" for function "calendr_events" is deprecated since version 1.21. Use Twig_SimpleFunction instead.

[yohang]

This shouldn't happen on Calendar 2.0+ as it's fixed since 2015.

a783d14#diff-1a006a43d4483ed3e84221340a01c3b8R32