Fixed gemspec errors

Hi,
Bundler (version 1.5.3) shows an error during toastr-rails installation process.

```
toastr-rails at .../.rvm/gems/ruby-2.0.0-p353/bundler/gems/toastr-rails-71a729d75d9d did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on bundler (>= 1.1.3, development), (>= 1.1.3) use:
    add_runtime_dependency 'bundler', '>= 1.1.3', '>= 1.1.3'

```