Change extension version name from number to string.

The number had no specific meaning it wasn't matching release version,
creating confusion.

Author: cristianberneanu

pg_diffix--1.0.0.sql pg_diffix--fir.sqlpg_diffix--1.0.0.sql renamed to pg_diffix--fir.sql

@@ -1,4 +1,4 @@
 comment = 'Open Diffix PostgreSQL extension'
-default_version = '1.0.0'
+default_version = 'fir'
 module_pathname = '$libdir/pg_diffix'
-schema = diffix
+schema = diffix