update exchange cache from 12->6 hours

src/main/java/com/danielbohry/stocks/config/CacheConfig.java

@@ -7,7 +7,7 @@ import org.springframework.cache.caffeine.CaffeineCacheManager;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
-import java.util.concurrent.TimeUnit;
+import static java.util.concurrent.TimeUnit.HOURS;
 
 @Configuration
 @EnableCaching
@@ -16,7 +16,7 @@ public class CacheConfig {
     @Bean
     public CacheManager cacheManager() {
         CaffeineCacheManager cacheManager = new CaffeineCacheManager("exchangeRates");
-        cacheManager.setCaffeine(Caffeine.newBuilder().expireAfterWrite(12, TimeUnit.HOURS));
+        cacheManager.setCaffeine(Caffeine.newBuilder().expireAfterWrite(6, HOURS));
         return cacheManager;
     }