Super-fast Travis JRuby install

.travis.yml

# Background: JRuby installs on Travis is painfully slow. The version which is preinstalled is very old. Here are some GitHub
# issues about it:
#
# https://github.com/travis-ci/docs-travis-ci-com/pull/743
# https://github.com/travis-ci/travis-rubies/issues/22
# https://github.com/travis-ci/travis-ci/issues/6892
# https://github.com/travis-ci/packer-templates/issues/391
#
# Since the upstream issue is still unresolved, and a JRuby install takes anywhere from 100 to 200 seconds (!), we decided to
# implement this workaround. With this in place, the JRuby install takes about 10 seconds.

sudo: false
language: ruby

# Dummy version to avoid wasting time on RVM installs.
rvm: 1.9.3

before_install:
  - if [ ! -d "$HOME/jruby-bin" ]; then mkdir -p ~/jruby-bin && pushd ~/jruby-bin && curl -o- -L https://s3.amazonaws.com/jruby.org/downloads/9.1.8.0/jruby-bin-9.1.8.0.tar.gz | tar xzf - --strip-components=1 && popd ; fi
  - export PATH="$HOME/jruby-bin/bin:$PATH"
  - gem install bundler

# The rest of your .travis.yml content goes here (run specs, build app etc).

Downside: breaks caching of Bundler packages (seems to use the Ruby version given in .rvm to perform the caching). Fixed in the latest commit.