Ignore .swp files.
Make sure output_stream can handle non-utf8 bytes

This is needed to safely output raw subunit v2 streams.
Move to subunit v2 for output of stored streams.

Passing --subunit to all testr commands will now consistently output
subunit v2. Previously it would output v1 for stored streams and v2
for live streams.
Make subunit 0.0.18 be a hard dependency.

The v2 protocol solves key issues in concurrency and stream handling.
Users that cannot use subunit v2 can run an older testrepository, or
contact upstream to work through whatever issue is blocking them.