[Sandbox] Try to make an educated guess as to where the MacPorts prefix is.

CHANGELOG.md

@@ -31,13 +31,9 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
   **NOTE**: _The above example is actually one that we know of. We’re not sure
   yet which process causes this, but there shouldn’t be a need for any process
   to read data from the root path anyways._
-
-  **NOTE 2**: _If you’re using a package manager other than Homebrew, please
-  submit a pull-request that adds the prefix of that manager
-  [here](https://github.com/CocoaPods/CocoaPods/blob/master/bin/sandbox-pod#L40)._
   
-  **NOTE 3**: _At the moment the sandbox is not compatible with the `:path` option 
-  when referencing Pods which are not stored within the directory of the Podfile._
+  **NOTE 2**: _At the moment the sandbox is not compatible with the `:path` option 
+  when referencing Pods that are not stored within the directory of the Podfile._
 
 * The naked `pod` command now defaults to `pod install`.
   [#958](https://github.com/CocoaPods/CocoaPods/issues/958)

bin/sandbox-pod

@@ -37,7 +37,11 @@ ruby_prefix = RbConfig::CONFIG['prefix']
 
 prefixes = ['/bin', '/usr/bin', '/usr/libexec']
 prefixes << `brew --prefix`.strip unless `which brew`.strip.empty?
-# TODO add MacPorts. More?
+# From asking people, it seems MacPorts does not have a `prefix` command, like
+# Homebrew does, so make an educated guess:
+unless (port = `which port`.strip).empty?
+  prefixes << File.dirname(File.dirname(port))
+end
 
 developer_prefix = `xcode-select --print-path`.strip
 xcode_app_path = File.expand_path('../..', developer_prefix)