pom.xml: update slf4j to 1.7.21

@see http://slf4j.org/news.html

No functional changes.

Author: php-coder

pom.xml

@@ -74,7 +74,7 @@
 		<h2.version>1.4.191</h2.version>
 		<mysql.version>5.1.38</mysql.version>
 		<servlet-api.version>3.1.0</servlet-api.version>
-		<slf4j.version>1.7.19</slf4j.version>
+		<slf4j.version>1.7.21</slf4j.version>
 		<spock.version>1.0-groovy-2.0</spock.version>
 		<thymeleaf.version>2.1.4.RELEASE</thymeleaf.version>
 		<thymeleaf-extras-springsecurity4.version>2.1.2.RELEASE</thymeleaf-extras-springsecurity4.version>