Fix rbenv gems helper

Fixes gem versions with words (beta, pre) in the version or with
multiple installed versions not being highlighted, and compatibility
with GNU sed (related to robbyrussell/oh-my-zsh#1579).

plugins/rbenv/rbenv.plugin.zsh

@@ -32,11 +32,11 @@ for rbenvdir in "${rbenvdirs[@]}" ; do
 
     function gems {
       local rbenv_path=$(rbenv prefix)
-      gem list $@ | sed \
-        -Ee "s/\([0-9\.]+( .+)?\)/$fg[blue]&$reset_color/g" \
-        -Ee "s|$(echo $rbenv_path)|$fg[magenta]\$rbenv_path$reset_color|g" \
-        -Ee "s/$current_ruby@global/$fg[yellow]&$reset_color/g" \
-        -Ee "s/$current_ruby$current_gemset$/$fg[green]&$reset_color/g"
+      gem list $@ | sed -E \
+        -e "s/\([0-9a-z, \.]+( .+)?\)/$fg[blue]&$reset_color/g" \
+        -e "s|$(echo $rbenv_path)|$fg[magenta]\$rbenv_path$reset_color|g" \
+        -e "s/$current_ruby@global/$fg[yellow]&$reset_color/g" \
+        -e "s/$current_ruby$current_gemset$/$fg[green]&$reset_color/g"
     }
 
     function rbenv_prompt_info() {