Use xcodebuild as default build tool and maybe add a task file

## Changes

* Swaps `xcodebuild` for `xctool` in `.travis.yml`
* Adds `xcpretty` as a build output formatter

## Rationale

* The build is not using the features of xctool like parallelization, and just needs a formatter

* We should avoid some of the compatibility issues between `xcodebuild` and `xctool` which may confuse new pod authors, like:
	* [The simulator failed to start, or the TEST_HOST application failed to run](https://github.com/facebook/xctool/issues/274)
	* [xctool fails to find schemes in sub-projects but xcodebuild works](https://github.com/facebook/xctool/issues/365)

## Discussion

* Not sure if these concerns override the [original rationale for using xctool](https://github.com/CocoaPods/pod-template/pull/54#discussion_r14232096) (not adding additional dependencies)

* This change doesn't add a task file ([Rakefile](https://github.com/AFNetworking/AFNetworking/blob/master/Rakefile) or [Makefile](https://github.com/kattrali/SimpleSyncService/blob/master/Makefile)), which is what I would actually do when making a pod. Important commands should not only committed to a service-specific file. Maybe we should add one?
	* If there is a task file, `xcodebuild` probably makes the most sense, since even if Travis has `xctool` by default, the average pod writer may not

Thoughts?
Maybe we should disable rdoc and ri documentation to speed this up?

```bash
$ gem install xcpretty --no-rdoc --no-ri --no-document --quiet
Successfully installed xcpretty-0.1.6
1 gem installed
```