gollum-lib linked to vulnerable nokogiri version

Hi,

`nokogiri` dependencies are tightened to `~> 1.6.4`, however 1.6.x includes vulnerable bundled libs:

```
Name: nokogiri
Version: 1.6.8.1
Advisory: CVE-2016-4658
Criticality: Unknown
URL: https://github.com/sparklemotion/nokogiri/issues/1615
Title: Nokogiri gem contains several vulnerabilities in libxml2 and libxslt
Solution: upgrade to >= 1.7.1

Vulnerabilities found!
```