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.

Author: ktdreyer

test/test_helper.rb

@@ -1,23 +1,27 @@
 unless defined?(JRUBY_VERSION)
-  require 'simplecov'
-  require 'coveralls'
-
-  SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
-    SimpleCov::Formatter::HTMLFormatter,
-    Coveralls::SimpleCov::Formatter
-  ]
-
-  SimpleCov.start do
-    project_name 'thread_safe'
-
-    add_filter '/examples/'
-    add_filter '/pkg/'
-    add_filter '/test/'
-    add_filter '/tasks/'
-    add_filter '/yard-template/'
-    add_filter '/yardoc/'
-
-    command_name 'Mintest'
+  begin
+    require 'simplecov'
+    require 'coveralls'
+
+    SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
+      SimpleCov::Formatter::HTMLFormatter,
+      Coveralls::SimpleCov::Formatter
+    ]
+
+    SimpleCov.start do
+      project_name 'thread_safe'
+
+      add_filter '/examples/'
+      add_filter '/pkg/'
+      add_filter '/test/'
+      add_filter '/tasks/'
+      add_filter '/yard-template/'
+      add_filter '/yardoc/'
+
+      command_name 'Mintest'
+    end
+  rescue LoadError
+    warn "warning: simplecov/coveralls gems not found; skipping coverage"
   end
 end