Fix SleipnirLoader to load spec files

[atermenji]

Usefull links from @AlexDenisov:

• https://www.reddit.com/r/iOSProgramming/comments/46muyc/diy_load_a_private_framework_at_runtime_to_use/
• https://gist.github.com/JohnCoates/94c25e19050ffa5ea981