Remove Gemfile.lock

The Gemfile.lock in a gem is only used during development, it is
arguably not essential to lock dependencies in development, and
having the file causes automatic security scanners to trigger
most often that not alerts that are not relevant.

I used to keep the file in place in order to get a heads up on
current security alerts in general, I guess it would be better
if I was doing so independently of this code when the alerts do not
actually affect it.

CHANGELOG.md

@@ -3,6 +3,12 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [Unreleased] - 2017-12-21
+
+## Fixed
+
+- Removed the `Gemfile.lock` - mostly to acknowledge that it was used only in development and is not really needed.
+
 ## [1.15.1] - 2017-01-26
 
 ## Fixed
@@ -268,6 +274,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 This [gist][gist] did refactor the Jose Valim's code into an `ActiveSupport::Concern`.
 
 [gist]: https://gist.github.com/gonzalo-bulnes/7659739
+[Unreleased]: https://github.com/gonzalo-bulnes/simple_token_authentication/compare/v1.15.1...master
 [1.15.1]: https://github.com/gonzalo-bulnes/simple_token_authentication/compare/v1.15.0...v1.15.1
 [1.15.0]: https://github.com/gonzalo-bulnes/simple_token_authentication/compare/v1.14.0...v1.15.0
 [1.14.0]: https://github.com/gonzalo-bulnes/simple_token_authentication/compare/v1.13.0...v1.14.0