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
Ignore tags file generated by ctags.
Don't update apt sources pointing to mirrors.

This change stops babushka from writing a new apt source file (and
running apt-get update) on ubuntu platforms where the apt source points
to a country mirror.

Example: I tend to configure apt sources to point to local (au) mirrors
before installing babushka on dev vms. Also recent vagrant boxes come
preconfigured with apt sources that look like this:

  deb http://us.archive.ubuntu.com/ubuntu natty main restricted

The "xx.archive" format unfortunately makes the met block in the
'apt source' dep fail.

Babushka should now recognise dep sources in the following formats:
http://archive.ubuntu.com/ubuntu
http://au.archive.ubuntu.com/ubuntu

For completeness I've checked that the following formats don't match:
http://wwwarchive.ubuntu.com/ubuntu
http://www.archive.ubuntu.com/ubuntu
http://a.archive.ubuntu.com/ubuntu
http://.archive.ubuntu.com/ubuntu
Don't update apt sources with trailing slashes.

Babushka's "apt source" met block currently fails if a trailing slash is
present in a dep source:

e.g. deb http://au.archive.ubuntu.com/ubuntu/ natty main restricted

I think the trailing slash on deb sources is more common than not. The
met block will now pass if a trailing slash exists.
Add Babushka::AptHelper.source_matcher_for_system.
Add debian support to #source_matcher_for_system.
Use #source_matcher_for_system in 'apt source' instead of regexifying the supplied URI.
Specs for matching ec2 deb sources.
Ubuntu source matcher now recognises ec2 archives.
Nest Asset#content_subdir specs within the main describe.
Add a missing spec: ensure Asset#extract yields the block.
Remove asset build dirs when the block is successful.
Capture STDERR output when checking versions with `cmd --version` (python is one example). Thanks to Tamir Duberstein.
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.