[core] Comments why Nokogiri 1.6 is not being used

The fog community decided to continue supporting Ruby 1.8.7 whilst
Nokogiri 1.6 drops support so ~> 1.5.0 is the correct version in
`fog.gemspec`.

See https://github.com/fog/fog/issues/1878 for more details.

[#1878]
Merge pull request #1891 from sferik/update_excon_dependency

Update excon dependency to version ~>0.24.0
Fix autoincrement when creating a flavor if private flavors exist.

Before, only public flavors were considered when finding the next free ID. That could lead to the case where the calculated ID was already taken which lead to a conflict. This happened whenever the user tried to create a private flavor via fog and then another (private OR public) flavor afterwards. Now, we also take private flavors into account, so the conflict cannot happen anymore.

As a side effect, it is now also possible to just return all private flavors, using the :is_public => false filter on the list_flavors_detail API call or the flavors collection.
Merge pull request #1894 from cloudbau/flavor_id_fix

Fix autoincrement when creating a flavor if private flavors exist.
[digitalocean] Rename do to docean in examples

Avoids using a reserved word, and `docean` is also used elsewhere in
the getting started guide.
Merge pull request #1901 from blom/patch-1

[digitalocean] Rename do to docean in examples
Update excon dependency to version ~>0.25.0