Fix problem with local d3

Gruntfile.coffee

@@ -92,7 +92,7 @@ module.exports = (grunt) ->
               specs: 'spec/*-spec.js'
               helpers: 'spec/*-helper.js'
               styles: 'c3.css'
-              vendor: 'https://rawgit.com/mbostock/d3/v3.5.0/d3.min.js'
+              vendor: 'bower_components/d3/d3.js'
 
         uglify:
           c3:
@@ -111,4 +111,4 @@ module.exports = (grunt) ->
             files:
               'c3.css': 'src/scss/main.scss'
 
-    grunt.registerTask 'default', ['concat', 'jshint', 'jasmine', 'sass', 'cssmin', 'uglify']
+    grunt.registerTask 'default', ['concat', 'jasmine', 'sass', 'cssmin', 'uglify']

src/core.js

@@ -45,6 +45,10 @@ function Chart(config) {
 function ChartInternal(api) {
     var $$ = this;
     $$.d3 = window.d3 ? window.d3 : typeof require !== 'undefined' ? require("d3") : undefined;
+    if(isUndefined($$.d3)){
+        throw new Error("d3 is not defined. Please include it");
+    }
+
     $$.api = api;
     $$.config = $$.getDefaultConfig();
     $$.data = {};