Add the ruby gem "certified" for additional CA sources

When connecting to HTTPS, the certs sometimes fail. One easy solution is to have the 'certified' gem as a dependency. This is easier than most other solutions and I guess it will occur pretty often when people use link-checker. Other solutions would require action from the actual user on this.

One example where it failed for me was github.

I tried writing a spec for the additional require (which is the only LOC change) but as all the specs are proper unit tests and there are no integration tests at the moment, I figured it'll be hard to emulate in a unit test. So, for now, no added tests. (some integration specs might be a nice addition)