Install curl through addons on Travis CI

.travis.yml

@@ -10,6 +10,10 @@ stages:
   # TODO: use this configuration instead to only build version tags
   # - name: release
   #   if: tag =~ ^v(\d+\.)*\d+$
+addons:
+  homebrew:
+    packages:
+    - curl
 
 jobs:
   include:
@@ -18,8 +22,6 @@ jobs:
       os: osx
       env: RUST_TARGET=x86_64-apple-darwin
       cache: cargo
-      install:
-        - brew install curl
       script:
         # TODO: remove after debugging
         - export TRAVIS_TAG=v0.2.18