fix task to run after vnum and installation

In Jubatus project, we tried to use `vnum` option when building shared library.
However, as `unittest_gtest` does not have dependency to `vnum`, unit tests run before `vnum` is processed (i.e., symbolic link to shared libraries is created.)
As a result, unit tests sometimes cause the link error in runtime:

```
jubatus/core/fv_converter/datum_to_fv_converter_test: error while loading shared libraries: libjubatus_core.so.0: cannot open shared object file: No such file or directory
```

This patch fixes `unittest_gtest` so that unit tests run after `vnum` and `install`.
This is the same dependency as `waf_unit_test`.

https://code.google.com/p/waf/source/browse/waflib/Tools/waf_unit_test.py#53