install valgrind on OSX

.travis.yml

@@ -35,6 +35,7 @@ matrix:
         homebrew:
           packages:
             - python@3.7
+            - valgrind
     - os: osx
       osx_image: xcode10.1
       compiler: gcc
@@ -43,6 +44,7 @@ matrix:
           packages:
             - gcc@8
             - python@3.7
+            - valgrind
       env:
         - MAKETARGET=test-all