Specify an explicit external encoding for tests

[TimMoore]

On my system, I have set my LC_ALL environment variable to C, because it speeds up a lot of common Unix commands when you're dealing with ASCII-only input.

This causes Ruby to use US-ASCII as the default external encoding, which triggers a failure in the Thor specs:

Failures:

  1) Thor::Shell::Basic#print_table uses maximum terminal width
     Failure/Error: expect(content).to eq(<<-TABLE)
     ArgumentError:
       invalid byte sequence in US-ASCII
     # ./spec/shell/basic_spec.rb:208:in `block (3 levels) in <top (required)>'

Changing Encoding.default_external globally in the spec helper is the most straightforward fix, but there are a lot of other possible ways to fix this, particularly by specifying an explicit encoding for the StringIO object created in the capture helper. Let me know if you'd like me to resubmit with a different solution.

[coveralls]

Coverage remained the same when pulling ed641ab on TimMoore:spec-encoding into 62593e0 on erikhuda:master.