Fix Maven warnings

[joschi]

[WARNING] Some problems were encountered while building the effective model for com.github.pinterest:kafkastats:jar:0.2.4.9
[WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-assembly-plugin @ line 154, column 21
[WARNING] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique: org.apache.avro:avro:jar -> duplicate declaration of version 1.8.2 @ com.github.pinterest:doctorkafka-parent:0.2.4.9, /Users/joschi/src/doctorkafka/pom.xml, line 101, column 15
[WARNING] 'build.plugins.plugin.version' for org.codehaus.mojo:build-helper-maven-plugin is missing. @ line 109, column 21
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-source-plugin is missing. @ line 142, column 21
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.github.pinterest:doctorkafka:jar:0.2.4.9
[WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-assembly-plugin @ line 263, column 12
[WARNING] 'build.plugins.plugin.version' for org.codehaus.mojo:build-helper-maven-plugin is missing. @ line 234, column 12
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-source-plugin is missing. @ line 251, column 12
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.github.pinterest:doctorkafka-parent:pom:0.2.4.9
[WARNING] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique: org.apache.avro:avro:jar -> duplicate declaration of version 1.8.2 @ line 101, column 15
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.