update JDK

ci_common/common.hocon

 # java 7 is needed by Truffle (for now)
 java7 : {name : oraclejdk, version : "7",    platformspecific: true}
 # java 8 must be a jvmci enabled variant
-java8 : {name : labsjdk, version : "8u172-jvmci-0.46", platformspecific: true}
+java8 : {name : labsjdk, version : "8u172-jvmci-0.47", platformspecific: true}
 java9 : {name : oraclejdk, version : "9.0.4+11", platformspecific: true}
 
 java8Downloads : {