Fix gradle classpath name

[w1nston]

The build script is naming the classpath with classpath gradle.plugin.com.etraveli.gradle.plugin:bom-verifier:1.0.1" that's one gradle.plugin too many.

[npetzall]

Might be a missing inside the plugin-plugin

mavenCoordinates {
  groupId = 'com.etraveli'  
  artifactId = 'bom-verifier'  
}

But this seems to require manual approval and that the plugin is published to bintray.
We should also remove the mavenCentral() repository since only jCenter is used when applied using the plugins api.