Updated package.json to use the correct casing for the 'main' entrypoint (angular#599)

In the rollup script 'angularFire2.umd.js' is used, where in the package.json 'angularfire2.umd.js' was used, causing for loading problems on linux based systems (Ubuntu in this case)

Author: maxmumford

rollup.publish.config.js

@@ -2,7 +2,7 @@ import globals from './rollup-globals';
 
 export default {
   entry: 'dist/index.js',
-  dest: 'dist/bundles/angularFire2.umd.js',
+  dest: 'dist/bundles/angularfire2.umd.js',
   format: 'umd',
   moduleName: 'angularFire2',
   globals