Installer: shared memory configuration for PostgreSQL

Fixes #450 - PostgreSQL fails to start.
Installer: fault-tolerant downloads

Fixes #433 - rvm installation is fragile; #441 - Retry mozroots during installation. Implements an easy-to-use "download" function and retry ability for running commands with a backoff policy.
Installer: handle exceptions when running commands
Merge pull request #452 from fernandoacorreia/installer-fixes

Installer fixes