Only append to paths file when using md5deep
Only append to paths file when using md5deep

Fixes https://github.com/travis-ci/travis-ci/issues/4203

There are a couple of issues when multiple directories are specified.

1. travis-build invokes `casher add` for each directory
2. `add` truncates the paths file on each invocation

We fix this issue by correctly appending to the paths file
for each invocation of `casher`.

travis-ci/casher,travis-ci/casher