Use `which` instead of `type -t` to detect a command.

Apart from "-t" not being supported by zsh's `type`, it does not appear
to work with bash, too.

Fixes: http://github.com/defunkt/hub/issues/#issue/47

Rakefile

@@ -5,7 +5,7 @@ require 'rake/testtask'
 #
 
 def command?(command)
-  `type -t #{command}`
+  `which #{command} 2>/dev/null`
   $?.success?
 end
 

lib/hub/commands.rb

@@ -542,7 +542,7 @@ help
     #
     # Returns a Boolean.
     def command?(name)
-      `type -t #{name}`
+      `which #{name} 2>/dev/null`
       $?.success?
     end