Reduce default processor count to two under CI environments

Most CI implementations like travis / circleci run the builds
containerised. This can lead to the situartion where many more
processors are detected via /proc/cpuinfo and friends than are actually
usable through resource limits enforced by the containerization.
This can result in high resource consumption that might
result in abnormal build termination.

The default can easily be overriden in your CI setup.

* http://fabiokung.com/2014/03/13/memory-inside-linux-containers/

lib/mutant.rb

@@ -28,6 +28,16 @@ module Mutant
 
   SCOPE_OPERATOR = '::'.freeze
 
+  # Test if CI is detected via environment
+  #
+  # @return [Boolean]
+  #
+  # @api private
+  #
+  def self.ci?
+    ENV.key?('CI')
+  end
+
   # Lookup constant for location
   #
   # @param [String] location
@@ -199,6 +209,8 @@ require 'mutant/zombifier/file'
 module Mutant
   # Reopen class to initialize constant to avoid dep circle
   class Config
+    CI_DEFAULT_PROCESSOR_COUNT = 2
+
     DEFAULT = new(
       debug:             false,
       fail_fast:         false,
@@ -209,7 +221,7 @@ module Mutant
       isolation:         Mutant::Isolation::Fork,
       reporter:          Reporter::CLI.build($stdout),
       zombie:            false,
-      processes:         Parallel.processor_count,
+      processes:         Mutant.ci? ? CI_DEFAULT_PROCESSOR_COUNT : Parallel.processor_count,
       expected_coverage: 100.0
     )
   end # Config

lib/mutant/reporter/cli.rb

@@ -13,10 +13,9 @@ module Mutant
       # @api private
       #
       def self.build(output)
-        ci = ENV.key?('CI')
         tty = output.respond_to?(:tty?) && output.tty?
         format =
-          if !ci && tty && Tput::INSTANCE.available
+          if !Mutant.ci? && tty && Tput::INSTANCE.available
             Format::Framed.new(tty:  tty, tput: Tput::INSTANCE)
           else
             Format::Progressive.new(tty: tty)

spec/unit/mutant_spec.rb

@@ -1,6 +1,18 @@
 RSpec.describe Mutant do
   let(:object) { described_class }
 
+  describe '.ci?' do
+    subject { object.ci? }
+
+    let(:result) { double('Result') }
+
+    before do
+      expect(ENV).to receive(:key?).with('CI').and_return(result)
+    end
+
+    it { should be(result) }
+  end
+
   describe '.zombify' do
     subject { object.zombify }