Fix for unknown symbol convert_to_bool 

PHP 5.4.9-4ubuntu2.4 
Running on 64-bit Ubuntu Raring

'PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/lib/php5/20100525/git2.so' - /usr/lib/php5/20100525/git2.so: undefined symbol: php_git2_git_diff_delta_to_array in Unknown on line 0'

Modified the one call for convert_to_bool to convert_to_boolean and everything works.  Checking the PHP source I don't see convert_to_bool in the 5.4 sourcecode