Add changelog entry for #9203 about schema dumper with db extensions

[ci skip]
Call super to use the abstract adapter implementation instead
Fix indentation of extensions in schema
Add blank line after extensions to separate from tables in schema
Do not print anything related to extensions when they don't none exist

When extensions are supported but there's no one enabled in the
database, we should not print anything related to them in schema.rb.
Run schema dumper extensions without creating real extensions

There is no need to create the extension in the database just to test if
it's dumped, we can stub that instead.
Skip schema dumper extensions test if connection does not support it

Travis failed because the postgresql version installed there does not
support extensions, so we just need to skip this for now.