don't set SSL_CERT_FILE for OSX

.travis.yml

@@ -45,7 +45,8 @@ install:
 
 script:
   # work around rust-lang/cargo#3340
-  - export SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt
+  - test "$TRAVIS_OS_NAME" = "osx" ||
+      export SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt
   - cargo generate-lockfile
   - if [[ $TRAVIS_OS_NAME = "linux" ]]; then
       sudo apt-get remove -y qemu-user-static &&