updated PLAI (and test cases) to use racket language rather than scheme

collects/plai/main.rkt

@@ -1,9 +1,9 @@
-#lang scheme
+#lang racket
 (require plai/datatype
          plai/test-harness)
 
 (provide (all-from-out plai/datatype)
-         (except-out (all-from-out scheme) error (for-syntax error) #%module-begin provide)
+         (except-out (all-from-out racket) error (for-syntax error) #%module-begin provide)
          (except-out (all-from-out plai/test-harness) plai-error)
          (rename-out [plai-error error] 
                      [plai-module-begin #%module-begin])

collects/tests/plai/web.rkt

@@ -4,4 +4,5 @@
   [mta])
 
 (define (start req)
-  "Hello")
+  (response/xexpr
+   "Hello"))