pom.xml should explicitly declare target java vesion

I had this problem:
```
[ERROR] /home/w/ua-parser/java/src/main/java/ua_parser/UserAgentParser.java:[31,20] generics are not supported in -source 1.3
[ERROR] (use -source 5 or higher to enable generics)
[ERROR] private final List<UAPattern> patterns;
[ERROR] 
```

My version of pom.xml add explicit declaration of target java version as 1.6 (can be changed to lower if needed)