Allow homebrew to be installed in non-standard locations

Closes #197

Author: adamwathan

cli/includes/compatibility.php

@@ -17,8 +17,8 @@
     exit(1);
 }
 
-if (exec('which brew') !== '/usr/local/bin/brew' && ! $inTestingEnvironment) {
-    echo 'Valet requires Brew to be installed on your Mac.';
+if (exec('which brew') == '' && ! $inTestingEnvironment) {
+    echo 'Valet requires Homebrew to be installed on your Mac.';
 
     exit(1);
 }