WIP: Replace parsing of `git` commands output with ruby-git library

[jamietanna]

Instead of parsing output of git commands, which are liable to change over time, we want to instead use a library, in this case ruby-git, which gives us a more straightforward interface.

Note that this is a first pass, and is NOT currently ready for merge - this is looking for early feedback. Currently outstanding:

• [ ] Timezone for date formats for commits
• [ ] Determining files changes in a commit
• [ ] Determining tracked files in the Git tree
• [ ] Tests are failing https://travis-ci.org/ivantsepp/jekyll-git_metadata/builds/438367044

Not sure whether in this PR or a follow-up, but perhaps refactoring the generation of the relevant data at #generate time to grab all the history details of the files in the repo and caching them, rather than iterating multiple times (which could be slow for a large repo).

As mentioned in #6, this will be a breaking change due to a change in how merge commits are dealt with.