Version bump to 1.2.4
Ignore `Gemfile.lock`.

It is not good practice to check in `Gemfile.lock`
for a gem. The only use of `Gemfile.lock` is for a
gem developer _or_ an automatic build/continuous
integration. And since travis-ci builds for different
platforms, a single `Gemfile.lock` actually works
counterproductive.
Regenerate gemspec for version 1.2.4