Use pessimistic version dependency for vcloud-core

Use Ruby's pessimistic operator[1] for the version dependency on the
vcloud-core gem, which prevents us from installing a version of
vcloud-core with a greater minor version than the one specified.

In this case, only versions less than 0.1.0 will be installed.

This reinforces our policy of semantic versioning, whereby a minor
version bump may signify that changes in that versions are not backwards
compatible.

Also bump version to 0.2.4 to encourage people to download this version
in anticipation of breaking changes to come soon.

[1]: http://guides.rubygems.org/patterns/#declaring_dependencies
Merge pull request #49 from alphagov/70500696-pessimistic-versions

[#70500696] Use pessimistic version dependency for vcloud-core
Add explicit namespace to Query and QueryRunner

Add the explicit namespacing to Query and QueryRunner in line with the
changes introduced in vcloud-core version 0.1.0, in which these classes
were moved under the Vcloud::Core namespace.

Also bumps the dependency on vcloud-core in the Gemspec so that we use
version 0.1.0, and bumps vcloud-edge_gateway to version 0.3.0.

Prefer a minor version bump for vcloud-edge_gateway (as opposed to a
patch version bump) to make it clearer that the changes in vcloud-core
broke backwards compatibility and that it's preferable to upgrade.

Depends on alphagov/vcloud-core#33.