experiment to disable duplicate cc test runs on osx and speed up travis
diff --git a/.travis/run.sh b/.travis/run.sh
index 4c01d67..76d0445 100755
--- a/.travis/run.sh
+++ b/.travis/run.sh
@@ -10,7 +10,12 @@
         export ARCHFLAGS="-arch x86_64"
         export LDFLAGS="-L/usr/local/opt/openssl/lib"
         export CFLAGS="-I/usr/local/opt/openssl/include"
+        # The Travis OS X jobs are run for two versions
+        # of OpenSSL, but we only need to run the
+        # CommonCrypto backend tests once. Exclude
+        # CommonCrypto when we test against brew OpenSSL
+        export TOX_FLAGS="-- --backend=openssl"
     fi
 fi
 source ~/.venv/bin/activate
-tox -e $TOX_ENV
+tox -e $TOX_ENV $TOX_FLAGS