Tests and refactors setting connection behaviour

Using fog, when using the compute service (confusingly also called
the connection) you should never need to be change the service a
resource is from.

This behaviour is deprecated after fog 0.8 so using a later version
begins outputting deprecation warnings when using the CLI.

This tests the methods in Brightbox::Account that were attempting this
behaviour. It appears to be a redundant operation.

Removes one use of Account#conn_account

After removing the attempt to set the service, #conn_account is the same
as @conn#account so removing to simply things.
Use latest upstream fog (v1.13.0)

Note: 0.13.0 is the last version of fog with support in the gemspec for
Ruby 1.8.7. 0.14 adds a hard dependency on Nokogiri 0.16
Replaces use deprecated accessor in specs

fog's #connection is deprecated since it should always have
been #service so the tests (to confirm that) are updated.
Locks nokogiri to the last 1.8.7 version

Nokogiri 1.6 drops Ruby 1.8.7 support and declares specifically in the
gemspec. We don't even USE nokogiri but it is a painful dependency of
fog.

So we declare the version we can work with.

We also nudge up to fog 1.14 which is identical except for a looser
nokogiri dependency that breaks bundling on 1.8.7 without the lock
above.
Merge pull request #34 from brightbox/update_fog

Updates fog to v1.14.0 (with locked Nokogiri)