Just a note in case anyone else has this problem:
We ran sudo pip install ansible==2.5.0.0 and tried to boot our vagrant box, but we kept running into this error: Trellis no longer supports Ansible 2.5.0. Please upgrade to Ansible 2.5.0.0 or higher.
Trellis was confused because our installed version was 2.5.0 while the REQUIRED version was 2.5.0.0, with 2 zeroes. To fix this I just edited the required version in trellis/lib/trellis/plugins/vars/version.py from 2.5.0.0 to 2.5.0, and the boot was successful.
I meant to post this earlier - I had the exact same problem after using sudo on the pip install command, although what fixed it for me was upgrading to ansible 2.5.1 instead