Revert "Replace available usage with type -q"

This reverts commit 6199f40.

I reverted this commit to support fish 2.0.0 properly, since `-q` option
of `type` function was introduced in fish 2.2.0.

See also:
  fish-shell/fish-shell@6f7a745
  fish-shell/fish-shell#1540
  fish-shell/fish-shell#54

fish_prompt.fish

@@ -236,6 +236,11 @@ function prompt_status -d "the symbols for a non zero exit status, root and back
     end
 end
 
+# `type -q` equivalent for fish<2.0.0
+function available -a name -d "Check if a function or program is available."
+  type "$name" ^/dev/null >&2
+end
+
 
 #
 # Prompt
@@ -246,8 +251,8 @@ function fish_prompt
   prompt_virtual_env
   prompt_user
   prompt_dir
-  type -q hg;  and prompt_hg
-  type -q git; and prompt_git
-  type -q svn; and prompt_svn
+  available hg;  and prompt_hg
+  available git; and prompt_git
+  available svn; and prompt_svn
   prompt_finish
 end