Merge pull request #2630 from reaperhulk/travis-update-osx

update osx_image values based on travis supported versions
diff --git a/.travis/run.sh b/.travis/run.sh
index 23b9e89..93dac33 100755
--- a/.travis/run.sh
+++ b/.travis/run.sh
@@ -15,6 +15,9 @@
         export LDFLAGS="/usr/local/opt/openssl/lib/libssl.a /usr/local/opt/openssl/lib/libcrypto.a"
     else
         export LDFLAGS="-L/usr/local/opt/openssl/lib"
+        # on a dynamic build we only need to test against OpenSSL -- CC is not affected by
+        # dynamic vs static
+        export TOX_FLAGS="--backend=openssl"
     fi
     export CFLAGS="-I/usr/local/opt/openssl/include"
 else