update jdk

ci.hocon

 java7 : {name : oraclejdk, version : "7",    platformspecific: true}
 #java8 : {name : oraclejdk, version : "8u66", platformspecific: true}
-java8 : {name : labsjdk, version : "8u92-jvmci-0.20", platformspecific: true}
+java8 : {name : labsjdk, version : "8u92-jvmci-0.21", platformspecific: true}
 logfiles : [
     "fastr_errors.log"
     "com.oracle.truffle.r.native/gnur/R-*/gnur_configure.log"