Initial fetch from NVD is too heavy (2.3 GB of memory consumed)

Fetching vulnerability data for the entire period from NVD was too heavy (It used  2.3GB of memory). 
I solved this problem.

see https://github.com/kotakanbe/go-cve-dictionary#usage-fetch-nvd-data

```
for i in {2002..2016}; do ./go-cve-dictionary fetchnvd -years $i; done
```

see also https://github.com/kotakanbe/go-cve-dictionary/pull/8
