The ornamental design for a shakercore, as shown and described.
FIG. 1 is a perspective view of a new design for a shaker core.
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of portions of the shaker core is included for the purpose of illustrating and forms no part of the claimed design. The shading is provided to illustrate contour only and is not an indication of texture, gloss, etc.