Bump slf4j-api from 1.8.0-beta2 to 1.8.0-beta4 (#4675)

Bumps [slf4j-api](https://github.com/qos-ch/slf4j) from 1.8.0-beta2 to 1.8.0-beta4.
- [Release notes](https://github.com/qos-ch/slf4j/releases)
- [Commits](https://github.com/qos-ch/slf4j/commits/v_1.8.0-beta4)

Signed-off-by: dependabot[bot] <support@dependabot.com>

build.gradle

@@ -146,7 +146,7 @@ dependencies {
     compile 'com.mashape.unirest:unirest-java:1.4.9'
 
     // >1.8.0-beta is required for java 9 compatibility
-    compile 'org.slf4j:slf4j-api:1.8.0-beta2'
+    compile 'org.slf4j:slf4j-api:1.8.0-beta4'
     compile 'org.apache.logging.log4j:log4j-slf4j18-impl:2.11.2'
     compile 'org.apache.logging.log4j:log4j-jcl:2.11.2'
     compile 'org.apache.logging.log4j:log4j-api:2.11.2'