RVM doesn't use the right Gem path: https://github.com/killerswan/ponyc/blob/f70cf38c8a246ff3ca6ec345736086ed0260ec83/.travis.yml#L82

travis.yml

rvm:
  - 2.2.3

install:
  - if [ "${TRAVIS_OS_NAME}" = "linux" ];
    then
      ...
      rvm use 2.2.3 --default;
      sudo gem install fpm;
    fi;

wrong-gem-path-error.log

Warning! PATH is not properly set up, '/usr/local/rvm/gems/ruby-2.2.3/bin' is not at first place,
         usually this is caused by shell initialization files - check them for 'PATH=...' entries,
         it might also help to re-add RVM to your dotfiles: 'rvm get stable --auto-dotfiles',
         to fix temporarily in this shell session run: 'rvm use ruby-2.2.3'.
Using /usr/local/rvm/gems/ruby-2.2.3
Fetching: json-1.8.3.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing fpm:
	ERROR: Failed to build gem native extension.
        /usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from extconf.rb:1:in `<main>'
Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.3 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.8.3/ext/json/ext/generator/gem_make.out

I'd think (in logs, line 6), that I've got the right Ruby set up now.

But when I call gem install fpm it's using the wrong ruby and failing badly.

This fixes it slightly!? (Now the install appears to work, but then FPM is missing when needed.)
https://travis-ci.org/killerswan/ponyc/jobs/148254231

And here:
https://travis-ci.org/killerswan/ponyc/jobs/148256203
https://github.com/killerswan/ponyc/blob/53082c2dd3917c435c5b5f8d6d8a2ecff59023a9/.travis.yml#L120

Aha, now on to a more complex problem:

Created package {:path=>"build/bin/ponyc_5f1e849_amd64.deb"}
Created package {:path=>"build/bin/ponyc-5f1e849-1.x86_64.rpm"}
fatal: Not a valid object name
make: *** [deploy] Error 128

Also, still:

[Bintray Upload] Reading descriptor file: bintray_debian.yml
/usr/local/rvm/gems/ruby-1.9.3-p551/gems/dpl-1.8.17/lib/dpl/provider/bintray.rb:54:in `read': No such file or directory - bintray_debian.yml (Errno::ENOENT)

https://travis-ci.org/killerswan/ponyc/jobs/148268944