add jcenter resolver

```
[warn] 	Note: Unresolved dependencies path:
[warn] 		com.github.bkromhout:java-diff-utils:2.1.1
[warn] 		  +- io.github.gitbucket:gitbucket_2.12:4.10.0 (/home/travis/build/gitbucket/gitbucket-gist-plugin/build.sbt#L12-17)
[warn] 		  +- io.github.gitbucket:gitbucket-gist-plugin_2.12:4.6.0
sbt.ResolveException: unresolved dependency: com.github.bkromhout#java-diff-utils;2.1.1: not found
	at sbt.IvyActions$.sbt$IvyActions$$resolve(IvyActions.scala:313)
```

https://travis-ci.org/gitbucket/gitbucket-gist-plugin/builds/205136180#L661

Author: xuwei-k

build.sbt

@@ -17,3 +17,5 @@ libraryDependencies ++= Seq(
 
 scalacOptions := Seq("-deprecation", "-feature", "-language:postfixOps", "-Ydelambdafy:method", "-target:jvm-1.8")
 javacOptions in compile ++= Seq("-target", "8", "-source", "8")
+
+useJCenter := true