Update to SQLite 3.15.1

Hi! SQLite 3.15.1[1], which is a bug-fix release, has been released. Therefore I have updated the bundled libsqlite in master and PHP 7.1. Running the test suites of ext/sqlite3 and ext/pdo_sqlite on Windows and Linux, both x64, didn't show any regression. Davey, Joe, if you're uncomfortable with this update so late in RC phase, please revert commit d6c36e9a[2]. [1] <http://sqlite.org/releaselog/3_15_1.html> [2] <http://git.php.net/?p=php-src.git;a=commit;h=d6c36e9af7ce43445b823e9a3b96868cf60fa7ed>