[bundler/bundler] Respect color option when instantiating shells

Thor's base shell will be memoized the first time it is set. So if we
instantiate a no-color shell first, further instantiations of a bundler
shell will be initialized with a no-color shell by default. This is
caused some sources specs to fail, depending on the order they run.

See for example https://travis-ci.org/bundler/bundler/builds/500328994.

What we do to fix it is to reset the shell unless no-color is explicitly
specified. That way, further instantiations will rerun thor's internal
logic to choose the default shell.

786b5d9894

lib/bundler/ui/shell.rb

@@ -10,9 +10,7 @@ module Bundler
       attr_writer :shell
 
       def initialize(options = {})
-        if options["no-color"]
-          Thor::Base.shell = Thor::Shell::Basic
-        end
+        Thor::Base.shell = options["no-color"] ? Thor::Shell::Basic : nil
         @shell = Thor::Base.shell.new
         @level = ENV["DEBUG"] ? "debug" : "info"
         @warning_history = []