CRJ-200 Captain reported encountering wake turbulence departing ATL in trail of a B757.

Narrative

Aircraft was cleared into position for takeoff. A 757 was departing from Runway 9L just prior to departure. Prior departing aircraft was not a heavy aircraft nor were they using heavy call sign. As soon as aircraft rotated and was clear of runway we were cleared for takeoff. A normal takeoff was executed and at V1 rotated at a normal rate. At approximately 200-300 ft. shaker activated. Flying Pilot immediately lowered nose and gained airspeed. Shaker went away and a normal climb was continued. Aircraft was at 200-300 ft. at an airspeed of approximately 160 KIAS with a V2 of 137 kts. when shaker activated. Continuous ignition did NOT activate and there was no indications of wake turbulence. No buffett or turbulence. Current winds [were] 055/11 at time of shaker. Aircraft departed 9L on an initial heading of 095 degrees. ATIS at time of departure was showing winds calm at surface. Aircraft recovered from shaker and proceeded to destination with no further incidents.Aircraft departed behind B-757. Aircraft did not have a 5 mile buffer and 2 min delay for wake turbulence.Crew should have delayed takeoff clearance from ATC and waited for 2 min or 5 miles. B-757 however was not a heavy aircraft nor did they use heavy call sign.