Fixing compiler warning for unused parameter.

either_stream.t.h

@@ -64,7 +64,7 @@ public:
   void test_either_propogates_error() {
     auto strm = stream::either::make_stream<MathObject>(1.0);
     strm.flatmap([](double d) { return divide(d, 0); })
-    .map([](double d) { TS_FAIL("Operating on bad data"); return 0; })
+    .map([](double) { TS_FAIL("Operating on bad data"); return 0; })
     .match([](double) { TS_FAIL("Expected Error Type"); },
            [](MathError e) { TS_ASSERT_EQUALS( MathError::DivideByZero, e ); });
   }