`SITerminalReporter` pretty-prints its logs if capable.

Using colors (ANSI escape sequences) and Unicode characters,
as supported by the environment and `stdout`.
`subliminal-instrument.sh` dynamically builds and runs `subliminal-instrument`.
`subliminal-test` uses `subliminal-instrument` to format `instruments`' output.
Ensure that `subliminal-instrument` will exit if/when `subliminal-test` is killed.
Update the Rakefile's documentation to reflect the supported test SDKs.
Run `subliminal-instrument`'s tests in CI.

This entails the addition of a new "CI_unit" rake task,
to run locally as part of the "test" task, gating the integration tests,
and to run as its own matrix entry on Travis.
DEBUG: `SIFileReportWriter` runs an "environment self-check".

To help figure out why the integration test runs aren't pretty-printing
the way we'd expect.
DEBUG: Set the TRAVIS environment variable.

The environment self-check appears to show that it is no longer set by Travis.