Update PrimitiveType.java

Author: Dichotomia

modules/swagger-core/src/main/java/io/swagger/util/PrimitiveType.java

@@ -201,6 +201,7 @@ public ObjectProperty createProperty() {
         addKeys(externalClasses, DATE, "org.joda.time.LocalDate", "java.time.LocalDate");
         addKeys(externalClasses, DATE_TIME, "org.joda.time.DateTime", "org.joda.time.ReadableDateTime",
                 "javax.xml.datatype.XMLGregorianCalendar", "java.time.LocalDateTime", "java.time.ZonedDateTime");
+        addKeys(externalClasses, LONG, "java.time.Instant");                
         EXTERNAL_CLASSES = Collections.unmodifiableMap(externalClasses);
 
         final Map<String, PrimitiveType> names = new TreeMap<String, PrimitiveType>(String.CASE_INSENSITIVE_ORDER);