Whitelist coveralls.io in webmock

Whitelist coveralls.io for webmock. While we do not use webmock
directly it is used by the fake_dropbox gem, hence the whitelist
code being in the only file that uses it.

If we end up with other gems that require webmock then we will
include it in our Gemfile directly and whitelist in the spec_helper

In the optimistic hope that this will fix the issue, lets also
add the Coveralls badge to the readme
Update gem dependencies