Bump ecmascript package version to 0.7.3.

Bumping this version is necessary after upgrading babel-compiler since
ecmascript registers a compiler plugin that uses babel-compiler.

packages/ecmascript/package.js

@@ -1,6 +1,6 @@
 Package.describe({
   name: 'ecmascript',
-  version: '0.7.2',
+  version: '0.7.3',
   summary: 'Compiler plugin that supports ES2015+ in all .js files',
   documentation: 'README.md'
 });