Change version to 1.1

Also clean up CHANGES.MD.

I'm opening this up as a PR because it's a switch to our new permanent versioning scheme and just wanted to make sure everyone is on the same page (and in agreement).

1. Cesium will primarily use a `major.minor` versioning scheme, where the minor version will be bumped with each monthly release; i.e. 12 months from now will be `Cesium 1.12`.
2. When releasing a out-of-band patch, (or when required by systems that follow semver) we will use a a third digit, i.e. `Cesium 1.12.1` would be an out of band patch to `1.12`.
3. Major version bumps will only be done when we feel it's necessary and not to indicate a breaking change.