Update README for sbt-js to mention SBT 0.12!

Thanks to Ben James for spotting this.
More Coffeescript options, altered Closure Compiler options:

  - Added `JsKeys.coffeeVersion` setting to switch back and forth between Coffeescript 1.1.0 and 1.6.1;

  - Added `JsKeys.coffeeBare` flag to toggle top-level function wrappers around Coffeescript files;

  - Renamed `JsKeys.optimisationLevel` to `JsKeys.compilationLevel` to be more in line with Google's nomenclature;

  - Altered the types of `JsKeys.warningLevel` and `JsKeys.compilationLevel` to introduce meaningful values,
    and introduced enumerations of possible values to make them easier to select.

  - Updated the `sbt-js` README to fix typos and document new features.
Upgrade to less 1.4.0.
Merge pull request #30 from devkat/master

Upgrade to less 1.4.0
Updated README after update to Less 1.4.0.

* Add new version to README file
* Removed version numbers from text to make it generic / future-proof
Merge pull request #31 from devkat/master

Updated README after update to Less 1.4.0