Rename Gemfile to gems.rb

In v2.0, Bundler will move from `Gemfile` (and `Gemfile.lock`) to
`gems.rb` (and `gems.locked`) as the default filenames for those files.
In the meantime, the new filenames are already supported by recent
releases of Bundler.

It's time to move boldly into the future.

Note that Bundler began acknowledging the new gem file format as of
v1.8, hence the `bundler` version requirement bump.

Reference: https://depfu.com/blog/2017/09/06/gemfiles-new-clothes

.gitignore

@@ -1,6 +1,6 @@
 *.gem
 .bundle
-Gemfile.lock
+gems.locked
 pkg
 bin/
 coverage/

.travis.yml

@@ -7,6 +7,8 @@ rvm:
 
 cache: bundler
 
+gemfile: gems.rb
+
 bundler_args: --without benchmark development
 
 before_install: gem update --system

Gemfile → gems.rb

@@ -17,7 +17,7 @@
 
 group :development do
   gem 'bump',    '~> 0.6.0', require: false
-  gem 'bundler', '~> 1.0',   require: false
+  gem 'bundler', '~> 1.8',   require: false
 end
 
 group :ci, :development do