Fix sed for OS X (#2080)

The sed command is a bit different in Mac OS X (why not!).

The ‘-i’ option required a parameter to tell what extension to add for the 
backup file. To fix it, just add extension for backup file, for example ‘.bak’ 

Fix broken Trevis CI test

No functional change.

.travis.yml

@@ -57,7 +57,7 @@ script:
   - make clean && make -j2 ARCH=x86-32 build && ../tests/signature.sh $benchref
 
   # Verify bench number is ONE_PLY independent by doubling its value
-  - sed -i 's/.*\(ONE_PLY = [0-9]*\),.*/\1 * 2,/g' types.h
+  - sed -i'.bak' 's/.*\(ONE_PLY = [0-9]*\),.*/\1 * 2,/g' types.h
   - make clean && make -j2 ARCH=x86-64 build && ../tests/signature.sh $benchref
   #
   # Check perft and reproducible search