fix warnings from Maven

[WARNING] Some problems were encountered while building the effective model for org.apache.orc:orc-shims:jar:1.5.0-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-compiler-plugin @ org.apache.orc:orc:1.5.0-SNAPSHOT, /Users/owen/work/code/orc/java/pom.xml, line 247, column 17
[WARNING]
[WARNING] Some problems were encountered while building the effective model for org.apache.orc:orc:pom:1.5.0-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-compiler-plugin @ line 247, column 17
[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.
[WARNING]
[WARNING] The project org.apache.orc:orc:pom:1.5.0-SNAPSHOT uses prerequisites which is only intended for maven-plugin projects but not for non maven-plugin projects. For such purposes you should use the maven-enforcer-plugin. See https://maven.apache.org/enforcer/enforcer-rules/requireMavenVersion.html