Specify an explicit external encoding for tests.

This fixes a failure in the spec at spec/shell/basic_spec.rb:208
(Thor::Shell::Basic#print_table uses maximum terminal width) when running on a
system with a default encoding that is not compatible with Unicode.

Author: TimMoore

spec/helper.rb

@@ -27,6 +27,7 @@
 $0 = 'thor'
 $thor_runner = true
 ARGV.clear
+Encoding.default_external = Encoding::UTF_8 if RUBY_VERSION > '1.8.7'
 Thor::Base.shell = Thor::Shell::Basic
 
 # Load fixtures