Try using system cmake

We were installing cmake 3.3.2, while the travis vms have 3.2.2, which
might be new enough.

.travis.yml

@@ -26,13 +26,6 @@ script:
     - set -e
     - export CC="gcc-4.8"
     - export CXX="g++-4.8"
-    - wget https://s3.amazonaws.com/download.draios.com/dependencies/cmake-3.3.2.tar.gz
-    - tar -xzf cmake-3.3.2.tar.gz
-    - cd cmake-3.3.2
-    - ./bootstrap --prefix=/usr
-    - make
-    - sudo make install
-    - cd ..
     - mkdir build
     - cd build
     - cmake .. -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DDRAIOS_DEBUG_FLAGS="-D_DEBUG -DNDEBUG"