Add NSCache category for subscripting awesomeness.

- That is you can now do the following with an `NSCache` instance:
`cache[theKey] = cacheMe;` and `id cachedObj = cache[theKey];`

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Change so that Travis only build the project.

- As we couldn't get Travis to run our unit tests I've reverted the config to
just build the project instead. I hope...

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>