Don't test tutorial_parallel.jl with Julia < 1.2

Author: tkf

test/test_examples_tutorial_parallel.jl

@@ -1,5 +1,10 @@
 module TestExamplesTutorialParallel
 
-include("../examples/tutorial_parallel.jl")
+# It looks like `Channel` was not thread-safe before 1.2.
+# * https://travis-ci.com/tkf/Transducers.jl/jobs/270458293
+# * https://github.com/JuliaLang/julia/commit/961907977a57ae7b72ddb374e63341f3633a0f0a
+if VERSION >= v"1.2"
+    include("../examples/tutorial_parallel.jl")
+end
 
 end  # module