Store the repositories into a thread-safe variable instead of a class variable

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Implements Spira.works_with

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Add Bundler Rake Tasks for build/install/release feature

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Rename Spira#works_with by Spira#using_repository

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Merge pull request #22 from PerfectMemory/feature-threadsafe

Threadsafe + Spira#using_repository