Upgrade to spring-boot 2.1.8

Fixes gh-377

Author: kazuki43zoo

pom.xml

@@ -72,7 +72,7 @@
     <mybatis-freemarker.version>1.2.0</mybatis-freemarker.version>
     <mybatis-velocity.version>2.1.0</mybatis-velocity.version>
     <mybatis-thymeleaf.version>1.0.1</mybatis-thymeleaf.version>
-    <spring-boot.version>2.1.7.RELEASE</spring-boot.version>
+    <spring-boot.version>2.1.8.RELEASE</spring-boot.version>
   </properties>
 
   <dependencyManagement>