Explicitly set fog-core version

[mattbostock]

We wouldn't normally specificy a fog-core version because it's already pulled in by fog [1](and thus not our concern).

But in this case, we set it explicitly so that we can ensure we use version 1.27.4 and above, which includes this bugfix:

fog/fog-core@58556e4

Without that bugfix, our integration tests (bundle exec rake integration) were failing with this error:

NoMethodError:
      undefined method `redisplay_progressbar' for Fog::Formatador:Class

Note that we change the version pinning slightly to use pessimistic versioning to prevent us from inadvertently using version 2 of fog-core, while ensuring that we use at least version 1.27.4.

See also the original bug report for vCloud Tools: https://groups.google.com/a/digital.cabinet-office.gov.uk/d/msg/vcloud-tools/GF-YrOVhUUU/bSQVpWyuaQkJ

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Note that currently, Fog master still requires only fog-core version 1.27.3 and above, which is why this error only occurred on some installations.

We raised a PR upstream against Fog to highlight the issue. It's currently failing on jruby but that's a separate issue: https://github.com/fog/fog/pull/3446

[mattbostock]

This change was merged upstream, so we remove the explicit pinning as soon as it's included in a release: https://github.com/fog/fog/pull/3446