Revert "Make Command and Path more string-like"

This reverts commit b3adc4c.

Turns out that this doesn't provide a lot of value, since a lot of
operations don't work on the Path and Command objects anyway, and it's
even somewhat misleading.

lib/vimrunner/command.rb

@@ -7,6 +7,5 @@ def initialize(command)
     def to_s
       @command.to_s.gsub("'", "''")
     end
-    alias_method :to_str, :to_s
   end
 end

lib/vimrunner/path.rb

@@ -7,6 +7,5 @@ def initialize(path)
     def to_s
       @path.to_s.gsub(/([^A-Za-z0-9_\-.,:\/@\n])/, "\\\\\\1")
     end
-    alias_method :to_str, :to_s
   end
 end

spec/vimrunner/command_spec.rb

@@ -9,9 +9,5 @@ module Vimrunner
     it "escapes single quotes" do
       Command.new("echo 'foo'").to_s.should eq("echo ''foo''")
     end
-
-    it "acts mostly as a string" do
-      ("one " + Command.new("two")).should eq('one two')
-    end
   end
 end

spec/vimrunner/path_spec.rb

@@ -9,9 +9,5 @@ module Vimrunner
     it "escapes non-standard characters in paths" do
       Path.new("foo bar!.txt").to_s.should eq('foo\ bar\!.txt')
     end
-
-    it "acts mostly as a string" do
-      ("one " + Path.new("two")).should eq('one two')
-    end
   end
 end