change language level for maven build plugin

```
[ERROR] (use -source 5 or higher to enable generics)
[ERROR] /home/dimzon/git/reflectasm/src/com/esotericsoftware/reflectasm/MethodAccess.java:[281,27] error: for-each loops are not supported in -source 1.3
[ERROR] 
[ERROR] (use -source 5 or higher to enable for-each loops)
[ERROR] /home/dimzon/git/reflectasm/src/com/esotericsoftware/reflectasm/FieldAccess.java:[4,7] error: static import declarations are not supported in -source 1.3
[ERROR] 
[ERROR] (use -source 5 or higher to enable static import declarations)
[ERROR] /home/dimzon/git/reflectasm/src/com/esotericsoftware/reflectasm/FieldAccess.java:[84,11] error: generics are not supported in -source 1.3
[ERROR] 
[ERROR] (use -source 5 or higher to enable generics)
[ERROR] /home/dimzon/git/reflectasm/src/com/esotericsoftware/reflectasm/AccessClassLoader.java:[14,33] error: generics are not supported in -source 1.3
[ERROR] 
[ERROR] (use -source 5 or higher to enable generics)
[ERROR] /home/dimzon/git/reflectasm/src/com/esotericsoftware/reflectasm/ConstructorAccess.java:[4,7] error: static import declarations are not supported in -source 1.3
```