Retry logic for CI

[lvh]

Builds have broken more times than I can count with errors like:

E: Failed to fetch http://httpredir.debian.org/debian/pool/main/l/llvm-toolchain-3.5/libllvm3.5_3.5-10_amd64.deb  Error reading from server. Remote end closed connection [IP: 5.153.231.35 80]

Example: https://travis-ci.org/RackSec/desdemona/jobs/116661802
#84 might solve this by running on a more reliable platform. If not, we should have some retry logic for this. For docker builds, sharing ~/.m2 and /var/cache/apt/archives might also make this error significantly less likely.