Switch to language_sniffer

Github's Linguist gem was very helpful, but the overhead
and number of dependencies for simple language detection
is unnecessary. language_sniffer is does all that we really
need.
Remove linguist references

Since `language_sniffer` does not support binary, image,
vendored, or generated info and we are not calculating
anything against it, we are removing it for now. If
`language_sniffer` ever supports this info, we will re-add
it back into the JSON output.
Merge pull request #57 from kevinjalbert/switch-to-language_sniffer

Switch to language_sniffer