Temporarily rm vendorized tsenart/tb
Squashed '_vendor/src/github.com/tsenart/tb/' content from commit cf767b3

git-subtree-dir: _vendor/src/github.com/tsenart/tb
git-subtree-split: cf767b3cc5ce58ab319e2393d9fead48061be7d3
Merge commit '940a4ff39a775b5c505d4d9c830e4f99ae2db9c6' as '_vendor/src/github.com/tsenart/tb'
roshi-walker: use new tb.Bucket API