Upgrade Spring 4.3.16.RELEASE -> 4.3.20.RELEASE (#31)

See:
- https://spring.io/blog/2018/04/05/multiple-cve-reports-published-for-the-spring-framework
- https://spring.io/blog/2018/06/13/spring-framework-5-0-7-and-4-3-18-available-now
- https://spring.io/blog/2018/09/07/spring-framework-5-1-rc3-5-0-9-and-4-3-19-available-now
- https://spring.io/blog/2018/10/15/spring-framework-5-1-1-5-0-10-and-4-3-20-available-now
- https://jira.spring.io/projects/SPR/versions/16751
- https://jira.spring.io/projects/SPR/versions/16805
- https://jira.spring.io/projects/SPR/versions/16939
- https://jira.spring.io/projects/SPR/versions/17116

pom.xml

@@ -191,7 +191,7 @@
             <dependency>
                 <groupId>org.springframework</groupId>
                 <artifactId>spring-framework-bom</artifactId>
-                <version>4.3.16.RELEASE</version>
+                <version>4.3.20.RELEASE</version>
                 <type>pom</type>
                 <scope>import</scope>
             </dependency>