Fix pubspec version constraints

pubspec.yaml

@@ -4,9 +4,9 @@ author: Anders Forsell <[email protected]>
 description: chessboard.dart is derived from chessboard.js, but created as a Polymer element (web component) that provides an easy way to include a chessboard in a web application.
 homepage: https://github.com/andersforsell/chessboard.dart
 dependencies:
-  chess: any
-  core_elements: any
-  paper_elements: any
+  chess: '>=0.6.1 <0.7.0'
+  core_elements: '>=0.0.6 <0.1.0'
+  paper_elements: '>=0.0.1 <0.1.0'
   polymer: '>=0.11.0-dev.2 <0.12.0'
 dev_dependencies:
   ghpages_generator: any