Function rb_exsit is renamed to rb_exists in v1.0, which prevents older version to be installed

[harry-leq]

Hi.
There's an issue while rb_exsit was renamed to rb_exists in v1.0. Even though there is a function replacement in roaringbitmap--0.5--1.0.sql, it's not enough.
I have raised a fix. More details are described in the PR: Function rb_exsit is renamed to rb_exists in v1.0.

[ChenHuajun]

Why use a higher-version .so file to create a lower-version extension? This behavior is not supported.