FIX: Fixed some issues around vb_suite
I'm new to vb_suite, and ran following command according to [this document](https://github.com/pydata/pandas/wiki/Performance-Testing).

```
./test_perf.sh -b master -t HEAD
```

But I got some errors (`InvalidGitRepositoryError` and Benchmark duplication errors), and finally I got tests run by fixing following issues.
- VB_DIR pointed wrong directory
- temporary variable `bmark` in groupby was collected as Benchmark
- indexing module had same name ('series_getitem_scalar') Benchmarks

I'm not so familier with vb_suite, and any feedbacks are welcome ! Thanks.
