Ensure that the test environment doesn't leak into the tests

This should get the build green in CI again.
Only use the pretty formatter when attached to a tty
Specs for matching ec2 deb sources.
Ubuntu source matcher now recognises ec2 archives.
Case-insensitive regex to detect Location header
Remove the (currently broken) '--track-blocks' option.
Merge branch 'asset-cleanup' -- remove build dirs on success.
Speed up two dep_specs by stubbing Levenshtein, which is expensive.
Use fuubar when profiling the specs.
Don't include any helpers when running the specs.
Call helpers directly on their modules as required to fix the specs.
Remove Object#taph.
Remove Source.sources_yml patch.
Add Source#remove! to Source with specs, instead of patching it on in spec_helper.
No need for utf-8 coding in spec_helper.
Call GitHelpers.git directly in Source#update!.
Stub GitHelpers.git as required instead of overriding Source.for_remote in spec_helper.
Whitespace.
Make the Babushka::DSL directly callable, and private when included.
Merge branch 'spec-cleanups'