Fix acts build for #21

#21 (closed) is still an issue (some symbols seem to be missing from the ACTS library), even after reverting the Eigen version. This needs urgent fixing. My best guess is that it is due to the complete overhaul of the package.py for acts together with the new releases. I'm trying to revert to an older version that worked in the past. Can't really think of anything else that makes this different from v2.3 (where everything works fine).

Closes #21 (closed)