[Podspec] Use vendored_libraries

[`$PODS_ROOT`](https://github.com/CocoaPods/CocoaPods/blob/master/CHANGELOG.md#breaking-1) is deprecated and you should be using `vendored_libraries` instead of `$PODS_ROOT` with `LIBRARY_SEARCH_PATHS` and `OTHER_LDFLAGS`.

The current podspec would break if it was used by a user from a directory with a space, because the path for `LIBRARY_SEARCH_PATHS` is not escaped if the value of `$PODS_ROOT` was to contain a space. This would also cause issues if the podspec was used via `pod 'Intercom', :path => '..etc...'` due to the path of Intercom inside `$PODS_ROOT` would be different.

**Note**: *I have not tested this other than with `pod lib lint`, please ensure that your own tests still pass.*