Fix previous patch for OS X (#961)

Use posix version of sed that is available on all
platforms.

No functional change.

.travis.yml

@@ -48,7 +48,7 @@ script:
   # checking bench for various build types
   #
   # obtain reference from git log
-  - git log | grep "\b[Bb]ench[ :]\+[0-9]\{7\}" | head -n 1 | sed -rn "s/[^0-9]*([0-9]+)/\1/p" > git_sig
+  - git log | grep "\b[Bb]ench[ :]\+[0-9]\{7\}" | head -n 1 | sed "s/[^0-9]*\([0-9][0-9]*\)/\1/g" > git_sig
   - export benchref=$(cat git_sig)
   - echo "Reference bench:" $benchref
   # verify against reference