Upgrade to SLF4J 1.7.8

Closes gh-2151

Author: BigMichi1

spring-boot-dependencies/pom.xml

@@ -94,7 +94,7 @@
 		<reactor.version>1.1.5.RELEASE</reactor.version>
 		<reactor-spring.version>1.1.3.RELEASE</reactor-spring.version>
 		<servlet-api.version>3.0.1</servlet-api.version>
-		<slf4j.version>1.7.7</slf4j.version>
+		<slf4j.version>1.7.8</slf4j.version>
 		<snakeyaml.version>1.13</snakeyaml.version>
 		<solr.version>4.7.2</solr.version>
 		<spock.version>0.7-groovy-2.0</spock.version>