Changed brew cmd to properly report if awscli is installed via homebrew

plugins/aws/aws.plugin.zsh

@@ -3,19 +3,17 @@ _homebrew-installed() {
 }
 
 _awscli-homebrew-installed() {
-  brew --prefix awscli &> /dev/null
+  brew list awscli &> /dev/null
 }
 
 export AWS_HOME=~/.aws
 
 function agp {
   echo $AWS_DEFAULT_PROFILE
-  
 }
 function asp {
   export AWS_DEFAULT_PROFILE=$1
   export RPROMPT="<aws:$AWS_DEFAULT_PROFILE>"
-    
 }
 function aws_profiles {
   reply=($(grep profile $AWS_HOME/config|sed -e 's/.*profile \([a-zA-Z0-9_-]*\).*/\1/'))