Add workaround for ruby 2.2

In ruby 2.2, TracePoint#binding may return nil.

Close #10.

Author: k-tsj

lib/power_assert.rb

@@ -118,6 +118,7 @@ def initialize(assertion_proc_or_source, assertion_method, source_binding)
       target_thread = Thread.current
       @trace = TracePoint.new(:return, :c_return) do |tp|
         next if method_ids and ! method_ids.include?(tp.method_id)
+        next unless tp.binding # workaround for ruby 2.2
         locs = tp.binding.eval('::Kernel.caller_locations')
         current_diff = locs.length - @base_caller_length
         target_diff = TARGET_CALLER_DIFF[tp.event]

test/test_power_assert.rb

@@ -413,4 +413,10 @@ def test_assertion_message_with_string
       String(a) + String(@b) + String(@@c) + String($d)
 END
   end
+
+  def test_workaround_for_ruby_2_2
+    assert_nothing_raised do
+      assertion_message { Thread.new {}.join }
+    end
+  end
 end