Revert "Install bundle gems to the system not to vendor/bundle"

This reverts commit 875dd3edfd34eaa20120b6589b0d1905230e948f.

The issue was fixed in rbx-2.2.3
Remove deprecation warning for FIXTURES_PATH

FIXTURES_PATH has a bit different case than DatabaseTasks.fixtures_path,
which was added along with deprecation. A use case for FIXTURES_PATH
could be loading fixtures from a different directory just for a given
test run. The implementation is fairly simple, so leaving it as it is
won't hurt.
Add a missing changelog entry for #13534 [ci skip]
Merge pull request #13538 from robin850/patch-14

Add a missing changelog entry for #13534 [ci skip]
tidy AR CHANGELOG. [ci skip]