Lower requirements down to 4.3.11 again (#1491)

There was an update with the syntax-highlighting module which fixed compatibility with 4.3.11.

README.md

@@ -9,7 +9,7 @@ Installation
 ------------
 
 Prezto will work with any recent release of Zsh, but the minimum required
-version is 4.3.17.
+version is 4.3.11.
 
   1. Launch Zsh:
 

init.zsh

@@ -10,7 +10,7 @@
 #
 
 # Check for the minimum supported version.
-min_zsh_version='4.3.17'
+min_zsh_version='4.3.11'
 if ! autoload -Uz is-at-least || ! is-at-least "$min_zsh_version"; then
   printf "prezto: old shell detected, minimum required: %s\n" "$min_zsh_version" >&2
   return 1