Fix maven warnings about versions and encoding

[robinst]

Fixes the following warnings when running mvn clean install:

[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.datadoghq:java-dogstatsd-client:jar:2.3
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-gpg-plugin is missing. @ line 116, column 15
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 96, column 15
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-javadoc-plugin is missing. @ line 104, 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.
[WARNING]

And:

[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!

[masci]

Thanks for fixing this! 🙇