Fix Travis CI

This commit is supposed to fix the following error on Travis:

```
4: from /home/travis/.rvm/gems/ruby-2.6.3/bin/ruby_executable_hooks:24:in `<main>'
  3: from /home/travis/.rvm/gems/ruby-2.6.3/bin/ruby_executable_hooks:24:in `eval'
  2: from /home/travis/.rvm/rubies/ruby-2.6.3/bin/bundle:23:in `<main>'
  1: from /home/travis/.rvm/rubies/ruby-2.6.3/lib/ruby/site_ruby/2.6.0/rubygems.rb:294:in `activate_bin_path'
/home/travis/.rvm/rubies/ruby-2.6.3/lib/ruby/site_ruby/2.6.0/rubygems.rb:275:in `find_spec_for_exe':
  Could not find 'bundler' (1.17.3) required by your /home/travis/build/plataformatec/simple_form/Gemfile.lock.
    (Gem::GemNotFoundException)
```

Ref: rubygems/bundler#6882 (comment)

Gemfile.lock

@@ -297,6 +297,3 @@ DEPENDENCIES
   rubinius-developer_tools
   rubysl (~> 2.0)
   simple_form!
-
-BUNDLED WITH
-   1.17.3