Naming version as upcoming one with "-pre" suffix.

This is how semver marks pre-release versions.

@mkroehnert It is probably good idea to always have "-pre"
and do the release process script with that in mind
(just remove "-pre", publish, increment and add "-pre").

IIRC this is how node does it; other projects probably, too.

@NicolasPetton Maybe there are more places where version is denoted, there it also should be changed.

bower.json

@@ -1,6 +1,6 @@
 {
   "name": "amber",
-  "version": "0.11.0",
+  "version": "0.12.0-pre",
   "main": "support/amber.js",
   "ignore": [
     "**/.*",

package.json

@@ -1,6 +1,6 @@
 {
   "name": "amber",
-  "version": "0.11.0",
+  "version": "0.12.0-pre",
   "description": "An implementation of the Smalltalk language that runs on top of the JS runtime.",
   "homepage": "http://amber-lang.net",
   "keywords": [