Fix for Issue #104

This fixes #104 by using [`CFNumberType`](https://developer.apple.com/library/mac/documentation/CoreFoundation/Reference/CFNumberRef/Reference/reference.html#//apple_ref/doc/uid/20001209-CH2g-C007966) instead of `@encode`.

I've also [update](https://github.com/rhodgkins/encrypted-core-data/commit/84ee471b5291898d99be0f84cc042c69e34784b0) the Travis CI info to build and test against both 64-bit and 32-bit. This means that I managed to produce some [test cases](https://github.com/rhodgkins/encrypted-core-data/commit/10b6844eac59c322fc0778bd2ed6f801ffda821e) to catch the error.