tests: make simplecov/coveralls optional

If we do not have SimpleCov or Coveralls installed, we should be able to
continue with the rest of the test suite.

This allows the tests to run outside of Bundler if SimpleCov or
Coveralls are not installed.

spec/spec_helper.rb

@@ -1,20 +1,24 @@
-require 'simplecov'
-require 'coveralls'
+begin
+  require 'simplecov'
+  require 'coveralls'
 
-# On Ruby 1.9+ use SimpleCov and publish to Coveralls.io
-if !RUBY_VERSION.start_with? '1.8'
-  SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
-    SimpleCov::Formatter::HTMLFormatter,
-    Coveralls::SimpleCov::Formatter
-  ]
-  SimpleCov.start do
-    add_filter '/spec/'    # exclude test code
-    add_filter '/vendor/'  # exclude gems which are vendored on Travis CI
+  # On Ruby 1.9+ use SimpleCov and publish to Coveralls.io
+  if !RUBY_VERSION.start_with? '1.8'
+    SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
+      SimpleCov::Formatter::HTMLFormatter,
+      Coveralls::SimpleCov::Formatter
+    ]
+    SimpleCov.start do
+      add_filter '/spec/'    # exclude test code
+      add_filter '/vendor/'  # exclude gems which are vendored on Travis CI
+    end
+
+    # Remove Docile, which was required by SimpleCov, to require again later
+    Object.send(:remove_const, :Docile)
+    $LOADED_FEATURES.reject! { |f| f =~ /\/docile\// }
   end
-
-  # Remove Docile, which was required by SimpleCov, to require again later
-  Object.send(:remove_const, :Docile)
-  $LOADED_FEATURES.reject! { |f| f =~ /\/docile\// }
+rescue LoadError
+  warn 'warning: simplecov/coveralls gems not found; skipping coverage'
 end
 
 lib_dir = File.join(File.dirname(File.dirname(__FILE__)), 'lib')
@@ -30,4 +34,4 @@ RSpec.configure do |config|
   config.expect_with :rspec do |c|
     c.syntax = :expect
   end
-end
+end