fix unix-crypt require syntax

[ryantm]

Bundle reported:

[!] There was an error parsing Gemfile: You passed
:require_name as an option for gem 'unix-crypt', but it is
invalid. Valid options are: group, groups, git, path, glob, name,
branch, ref, tag, require, submodules, platform, platforms, type,
source, install_if, gemfile, github, gist, bitbucket. You may be able
to resolve this by upgrading Bundler to the newest version.. Bundler
cannot continue.

Looks like maybe :require_name was ignored before, but now bundler is
validating the options passed to it.

This changes it to the correct way of naming the file to require.

[sebix]

I can confirm that the gem is also not installed automatically (because of this, I assume):

root@sebisplayground1-app:/var/www/discourse/plugins/discourse-migratepassword# gem install unix-crypt                                                                                                                                       
Fetching unix-crypt-1.3.1.gem                                                                                                                                                                                                                
Successfully installed unix-crypt-1.3.1                                                                                                                                                                                                      
1 gem installed                                                                                                                                                                                                                              

@communiteq Can you please merge this PR to fix the unix-pw support?

[sebix]

I can confirm that the gem is also not installed automatically (because of this, I assume):

OK, in later runs the plugin worked fine.