Rename Section::yield method to do_yield for compatibility with PHP 5.5

In PHP 5.5 yield is a reserved keyword
http://www.php.net/manual/en/language.generators.syntax.php#control-structures.yield
Use Section::do_yield in Blade::compile_yields method
Merge pull request #4 from relu/master

Rename Section::yield method to do_yield for compatibility with PHP 5.5