Make access_token_secret nullable

For now this column is not being used. 

It is mainly for other social providers that might be added in the future so it could be erased all together or made NULL by default.

Author: msurguy

app/database/migrations/2013_11_13_222813_create_profiles_table.php

@@ -22,7 +22,7 @@ public function up()
             $table->string('description')->nullable()->default(NULL);
             $table->string('image_url')->nullable()->default(NULL);
             $table->string('access_token')->nullable()->default(NULL);
-            $table->string('access_token_secret');
+            $table->string('access_token_secret')->nullable()->default(NULL);
             $table->timestamps();
 
             $table->foreign('user_id')