Their top 51 contracts total $138 million right now or $10 million under the 2015 cap. They'd have the room to take on Mathis' contract ($6.5 million cap hit) and do some roster pruning and restructuring after June 1.
Cutting Williams after 6/1 frees up $2.5 million with only $875,000 in dead money. Find another $4 million somewhere and you're good to go. Since they don't have to sign a first round rookie this year, that takes some pressure off as well.