Don't link RAC and Mantle into iOS library target

Consumers may have multiple copies of the nested dependencies, so they
need to link there. This is consistent with our README instructions.
Explicitly build Mantle and RAC before iOS library

This ensures that their headers are copied into place before trying to
compile OctoKit, but I'm worried about the effect this may have on the
dependency flattening work introduced in #176.

/cc @robrix
Merge branch 'fix-ios-linking' into rac-3.0