Fixed primary key too long identifier name

[rtorralba]

When index is primary the index name is always 'PRIMARY', then is not necessary to check isDefaultIndexName. If you don't set index name, try to make a index identifier joining (implode) all fields of the key and if this field is bigger than 64 characters it fail

[rtorralba]

I executed your project phpunit tests locally and fails too:

There was 1 error:

1. Xethron\MigrationsGenerator\MigrationsGeneratorServiceProviderTest::registers_migrations_generator
   BadMethodCallException: Method Mockery_0_stdClass::singleton() does not exist on this mock object

I can't access to the travis execution log for view what exactly fails.