When killing buck, escape the path to the pex

This is because we might well have spaces in the path
name. And, as we all know, spaces are the work of things
that are very, very evil when using build tools.

Fixed in the Fix a Committer, Become a Bug session at
Selenium Conf '16.

/face palm, one more time. Oh yeah!

rake-tasks/buck.rb

@@ -19,7 +19,7 @@ def self.download
 
       if cached_hash == out_hash
         # Make sure we're running a pristine buck instance
-        sh "python #{out} kill"
+        sh "python \"#{out}\" kill"
         return ["python", out]
       end
 
@@ -38,7 +38,7 @@ def self.download
 
       ant.get('src' => url, 'dest' => out, 'verbose' => true)
       File.chmod(0755, out)
-      sh "python #{out} kill"
+      sh "python \"#{out}\" kill"
       ["python", out]
     )
   end