Fix case for License.md file

To prevent

```
rspec-rails at /home/travis/build/rspec/rspec-rails 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:
  ["License.md"] are not files
```

Author: David Rodríguez

rspec-rails.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
   s.description = "rspec-rails is a testing framework for Rails 3.x and 4.x."
 
   s.files            = `git ls-files -- lib/*`.split("\n")
-  s.files           += %w[README.md License.md Changelog.md Capybara.md .yardopts .document]
+  s.files           += %w[README.md LICENSE.md Changelog.md Capybara.md .yardopts .document]
   s.test_files       = []
   s.rdoc_options     = ["--charset=UTF-8"]
   s.require_path     = "lib"