iPhone 6 and 6 Plus support

Antoine et al,

We're getting ready to re-submit Fetchnotes to include support for iOS 8 and noticed that the `launchimage` module does not yet support the recently announced iPhone 6 and 6 Plus device sizes. The necessary launch image sizes are:

* iPhone 6: `750x1334` (`667h@2x`)
* iPhone 6 Plus: `1242x2208` (`736h@3x`)

I forged ahead (ha!) and changed everything that seemed necessary to give `launchimage` support for these new sizes. Please let me know if I missed something! The changes are as follows (see diff also):

* [x] Device size checks in `launchimage_Util.m`
* [x] Added `750x1334` and `1242x2208` properties in `config_schema.json`
* [x] Updated `docs/index.md` to mention iPhone 6 and 6 Plus sizes
* [x] Copy launch image paths in `build_steps.json`
* [x] Added fixture images for `750x1334` and `1242x2208` sizes
* [x] Updated `inspector_config.json` to reflect these new fixtures
* [x] Updated `manifest.json` with version number and changes notes

edit: I should also mention that, because this is a public module managed by Trigger, I'm unable to test this at all. I suggest testing this with iPhone 6 and 6 Plus simulators before merging and/or updating the remote ForgeModule.