Fix ios:bootstrap error

`rake bootstrap` throws the following error while executing ios:bootstrap

```
pod install
Setting up CocoaPods master repo
Updating spec repo `master'

[!] The `master' repo requires CocoaPods 0.18.1 - 
Update Cocoapods, or checkout the appropriate tag in the repo.

rake aborted!
Command failed with status (1): [pod install...]
```