install sensible openssl on osx

.travis.yml

@@ -8,13 +8,13 @@ matrix:
       dist: trusty
       sudo: required
     - os: osx
-      osx_image: xcode8i
-  allow_failures:
-    - os: osx
 rust:
   - stable
   - beta
   - nightly
+before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update          ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install openssl ; fi
 script:
   - cargo build
   - cargo test