Failing osx travis builds: No space left on device

[jchodera]

The osx travis builds are failing frequently with

/Users/travis/.travis/job_stages: line 909: /var/folders/vy/rcv48w3j4w79llzf_x6qnvw40000gn/T/travis_tmp.ci9vWejy/pids_after: No space left on device
awk: can't open file /var/folders/vy/rcv48w3j4w79llzf_x6qnvw40000gn/T/travis_tmp.ci9vWejy/pids_after
 input record number 64, file /var/folders/vy/rcv48w3j4w79llzf_x6qnvw40000gn/T/travis_tmp.ci9vWejy/pids_after
 source line number 1
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

PR #885 attempted to eliminate this issue by cleaning up build artifacts after each package build, but appears to have been unsuccessful at eliminating these errors.