Add build matrix

.travis.yml

 language: ruby
-bundler_args: --without debug
-script: bundle exec rake test_with_coveralls
 sudo: false
 cache: bundler
-rvm:
-  - 2.3.0
-  - 2.2
-  - 2.1
-  - 2.0
-  - rbx-2
+matrix:
+  include:
+  - rvm: 2.3.1
+    env: WITH_COVERALLS=true
+  - rvm: 2.2
+    env: WITH_COVERALLS=false
+  - rvm: 2.1
+    env: WITH_COVERALLS=false
+  - rvm: 2.0
+    env: WITH_COVERALLS=false
+  - rvm: jruby-19mode
+    env: WITH_COVERALLS=false
+  - rvm: rbx-2
+    env: WITH_COVERALLS=false
+  allow_failures:
+  - rvm: rbx-2
+install:
+- if [[ $WITH_COVERALLS = "true" ]]; then
+    bundle install --without debug extra;
+  else
+    bundle install --without debug extra coverage;
+  fi
+script:
+- if [[ $WITH_COVERALLS = "true" ]]; then
+    bundle exec rake test_with_coveralls;
+  else
+    bundle exec rake;
+  fi
 notifications:
   email:
     recipients:
       - sylvester@keil.or.at
     on_success: change
-    on_failure: always
-matrix:
-  allow_failures:
-    - rvm: rbx-2
+    on_failure: change

Gemfile

@@ -2,12 +2,10 @@ source 'https://rubygems.org'
 gemspec
 
 group :development do
-
   if RUBY_VERSION >= '2.2.2'
     gem 'test-unit'
   else
     gem 'minitest', '< 5.0'
-    gem 'listen', '~>3.0.0'
   end
 
   gem 'rake'
@@ -17,7 +15,13 @@ group :development do
   gem 'redcarpet'
 
   gem 'unicode_utils'
+end
+
+group :extra do
+    gem 'listen', '~>3.0.0'
+end
 
+group :coverage do
   gem 'simplecov', '~>0.9', :require => false
   gem 'rubinius-coverage', :platform => :rbx
   gem 'coveralls', :require => false

Rakefile

@@ -15,9 +15,13 @@ Cucumber::Rake::Task.new(:features)
 
 task :default => [:features]
 
-require 'coveralls/rake/task'
-Coveralls::RakeTask.new
-task :test_with_coveralls => [:features, 'coveralls:push']
+begin
+  require 'coveralls/rake/task'
+  Coveralls::RakeTask.new
+  task :test_with_coveralls => [:features, 'coveralls:push']
+rescue LoadError => e
+  # ignore
+end
 
 task :release do |t|
   system "gem build jekyll-scholar.gemspec"