Simplify cache dependency (#34)
Simplify cache dependency (#34)

This Vault PHP client only uses the array cache adapter, so
there is no need to require the complete cache/cache package.

This change was prompted when trying to require the
contentful/contentful:6.0.3 package into a project where
csharpru/vault-php was already installed. Since the Contentful
package requires the void cache adapter, it cannot be installed
in a project with the complete cache/cache package already
installed. This is because cache/cache conflicts with all of the
individual cache/* packages.

Checked that all tests pass.
CSharpRU/vault-php