explicitly specified version for maven-compiler-plugin due to maven3 compatibility

When building project with maven3, this warning is observed.

```
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for com.endgame:storm-metrics-statsd:jar:1.0.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 49, 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] 
```

This is due to more strict rules in maven3, details [here](http://stackoverflow.com/questions/4123044/maven-3-warnings-about-build-plugins-plugin-version).

Specifying explicit version in pom.xml fixes this.