made svg reporter optional

`gh-badges` brings in *a lot* of native dependencies. Here's what using ass with travis requires now https://github.com/mozilla/fxa-auth-db-server/blob/d52ab1a7ca91cd4098089f978e3914d7d0d9b918/.travis.yml#L21

By just making `gh-badges` optional anyone can opt-out of all that.

fixes #7