Efficient query to get greatest value per group from big table
Index
A plain multicolumn B-tree index should work after all:
CREATE INDEX foo_idx
ON geoposition_records (equipment_id, created_at DESC NULLS LAST);

Why DESC NULLS LAST?

Unused index in range of dates query

It's safe to assume you have an equipment table? Then performance won't be a problem:
Correlated subquery
Based on this equipment table, run a lowly correlated subquery to great effect:
SELECT equipment_id
     ,(SELECT created_at
       FROM   geoposition_records
       WHERE  equipment_id = eq.equipment_id
       ORDER  BY created_at DESC NULLS LAST
       LIMIT  1) AS latest
FROM   equipment eq;

For a small number of rows in the equipment table (57 judging from your EXPLAIN ANALYZE output), that's very fast.
LATERAL join in Postgres 9.3+
SELECT eq.equipment_id, r.latest
FROM   equipment eq
LEFT   JOIN LATERAL (
   SELECT created_at
   FROM   geoposition_records
   WHERE  equipment_id = eq.equipment_id
   ORDER  BY created_at DESC NULLS LAST
   LIMIT  1
   ) r(latest) ON true;

Detailed explanation:

Optimize GROUP BY query to retrieve latest record per user

Performance similar to the correlated subquery.
Function
If you can't talk sense into the query planner (which shouldn't occur), a function looping through the equipment table is certain to do the trick. Looking up one equipment_id at a time uses the index.
CREATE OR REPLACE FUNCTION f_latest_equip()
  RETURNS TABLE (equipment_id int, latest timestamp)
  LANGUAGE plpgsql STABLE AS
$func$
BEGIN
   FOR equipment_id IN
      SELECT e.equipment_id FROM equipment e ORDER BY 1
   LOOP
      SELECT g.created_at
      FROM   geoposition_records g
      WHERE  g.equipment_id = f_latest_equip.equipment_id
                           -- prepend function name to disambiguate
      ORDER  BY g.created_at DESC NULLS LAST
      LIMIT  1
      INTO   latest;

      RETURN NEXT;
   END LOOP;
END  
$func$;

Makes for a nice call, too:
SELECT * FROM f_latest_equip();

Performance comparison:
db&lt;&gt;fiddle here
OLD sqlfiddle