Update gettext_i18n_rails_js.gemspec

fix case typo in gemspec

linux filename is case-sensitive

otherwise following warning when install it:

```
gettext_i18n_rails_js at /home/lidaobing/.rvm/gems/ruby-1.9.3-p327/bundler/gems/gettext_i18n_rails_js-5809a5727322 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:
  ["Readme.md"] are not files
```