Update warclight gem install branch to 'main'

[ato]

It appears rails new otherwise tries to use master branch which doesn't exist anymore:

$ rails _5.2.4.4_ new testapp -m https://raw.githubusercontent.com/archivesunleashed/warclight/main/template.rb
...
Fetching https://github.com/archivesunleashed/warclight.git
fatal: Needed a single revision
Git error: command `git rev-parse --verify master` in directory /tmp/testapp has failed.
Revision master does not exist in the repository https://github.com/archivesunleashed/warclight.git. Maybe you misspelled it?
If this error persists you could try removing the cache directory '/home/user/.bundle/cache/git/warclight-262a12efd637b38a13c36fcf8226a6b82a5a3a56'

[ruebot]

Good catch. Thanks @ato!