Fix a hidden dependency on the old V1 plugin API

ShapeShifter derived from `KaleidoscopePlugin`, which was part of the V1 plugin
API, and has been recently removed. Use `kaleidoscope::Plugin` instead, which is
its replacement.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/Kaleidoscope/ShapeShifter.h

@@ -22,7 +22,7 @@
 
 namespace kaleidoscope {
 
-class ShapeShifter : public KaleidoscopePlugin {
+class ShapeShifter : public kaleidoscope::Plugin {
  public:
   typedef struct {
     Key original, replacement;