[WIP] Fix Time + Update

**THIS IS WORK IN PROGRESS, DO NOT MERGE**

I got problems with my current project, so i decided to fix some things of the Gem now (and least for my cases) and ask you if you think that this changes could go into a Gem update.

#### 1. Fix (https://github.com/philipgiuliani/itunes-receipt/commit/2dd48709b3c00c282d813055ecc13008ce5f2e49)
Why should a object have methods like `request_date_ms `, `request_date_pst`? Apple returns them, but its definitely enough to parse the default `request_date` with `Time.parse` since Apple returns a string with the Timezone included.

In ruby you can than just say `receipt.purchase_date` and it will return it in the configured Timezone, if you want to get another one, you can say `receipt.purchase_data.in_time_zone("Rome")` for example.

This fix was applied to:
* purchase_date
* request_date
* expires_date

#### 2. Fix (https://github.com/philipgiuliani/itunes-receipt/commit/7788f63be9972a43f54f8ea518655d53d982ca75)
Since this projects is only Ruby 1.9+ (.travis.yml), i started with updating the Hash syntax to Ruby 1.9 Syntax.

#### TODO
* [ ] Update tests
  * [ ] Remove deprecations (RSpec 3 syntax)
  * [ ] Refactor
  * [ ] Remove unneeded tests

### Questions to @nov
I noticed that the Gem is very out-of-date with the new apple APIs. A lot of the [Receipt fields](https://developer.apple.com/library/ios/releasenotes/General/ValidateAppStoreReceipt/Chapters/ReceiptFields.html) aren't existing anymore and new has been added.

What do you think about updating it an doing a Major Version bump?