Lock rake to 10.1 for Ruby 1.8.7

Rake has gone 1.9.3+ from 10.2 onwards so the Gemfile for 1.8.7
specifies this new constraint.
Merge pull request #2786 from fog/fix_rake_on_187

Lock rake to 10.1 for Ruby 1.8.7
[Vsphere] Isolate helper from core

`Fog.class_from_string` was moved to core but is only used by the
VSphere provider with very little capacity for reuse.

This moves it to the `Fog::Vsphere` namespace so we can delete it from
core.
Merge pull request #2787 from fog/isolate_vsphere_helper

[Vsphere] Isolate helper from core
Move 1.8.7's Gemfile for subdirectory

Primarily just for Travis CI this is moved to reduce the top level files
when other testing Gemfiles are added.
Fix reference to fog's gemspec
Test the latest edge version of fog and parts

This uses git references to test fog against the latest repos for
extracted parts such as core and providers.
Merge pull request #2789 from fog/edge_gemfile

Adds Gemfile for Travis to test against edge versions of dependencies
[GH-1390] Remove redundant calls to Fog.credentials

If you use the supported means to create services such as:

    Fog::Service.new(:provider => "Example")

It already loads the contents of `~/.fog` and filters the values based
on if they are recognised or required.

That means in a number of services that are initialising based on
`options` or a value from `Fog.credentials` both are the same.

The `options` should have been loaded and merged from credentials in the
service "builder".