Fix travis build + build warnings

The cmake download was failing, so we were relying on the old cmake,
which I broke recently (in 7ae96c710b).
Now the download should work again.

Also fixed warning that the requested OSX image no longer exists and
that it is automatically substituted for xcode 7.3 image.

.travis.yml

@@ -119,12 +119,12 @@ matrix:
 
     # 3/ OSX Clang Builds
     - os: osx
-      osx_image: xcode7
+      osx_image: xcode7.3
       compiler: clang
       env: COMPILER='ccache clang++' BUILD_TYPE='Debug'
 
     - os: osx
-      osx_image: xcode7
+      osx_image: xcode7.3
       compiler: clang
       env: COMPILER='ccache clang++' BUILD_TYPE='Release'
 
@@ -145,7 +145,7 @@ install:
   - |
     if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
       CMAKE_URL="http://www.cmake.org/files/v3.3/cmake-3.3.2-Linux-x86_64.tar.gz"
-      mkdir cmake && travis_retry wget --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
+      mkdir cmake && travis_retry wget --no-check-certificate --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
       export PATH=${DEPS_DIR}/cmake/bin:${PATH}
     elif [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then
       which cmake || brew install cmake