[redpen-server]fix: set maven-compiler with source level 1.6 to build

In Ubuntu(java version 1.7.0_55, mvn version 3.0.5), `mvn install` failed to build redpen-server.
Errors below:
```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.0.2:compile (default-compile) on project redpen-server: Compilation failure: Compilation failure:
[ERROR] /home/redpen/redpen/redpen-server/src/main/java/org/bigram/docvalidator/server/api/DocumentValidateResource.java:[48,1] error: annotations are not supported in -source 1.3
[ERROR]
...
```
It is because `maven-compiler-plugin` default use source level 1.3.
So I added specified its source version `1.6`.
Thanks.