Filter `/vendor/bundle` by default

The Rails profile filters out `/vendor/bundle`, but this is needed
for many other kinds of Ruby projects.

A GitHub search illustrates how often developers need to
configure this:

https://github.com/search?utf8=%E2%9C%93&q=simplecov+%22add_filter*vendor%22&type=Code&ref=searchresults

I can't think of any downside of having this as the default behaviour.