Use local path to bootstrap.min.css

[AntonTkachev]

https://github.com/sksamuel/scapegoat/blob/243afff7d534b555e39807360ea30fb61072540a/src/main/scala/com/sksamuel/scapegoat/io/HtmlReportWriter.scala#L63

What you think about add constructor in this method with path to bootstrap.css?
In this case we can have opportunity work without internet because will use path to local bootstrap's file.

P.S.
If you agree with it, i can do pull request with this changes.