travis: enable CMake build

Because we support CMake 2.6.4 or later again.

.travis.yml

@@ -8,8 +8,7 @@ compiler:
 env:
   - BUILD_TOOL=autotools
   - BUILD_TOOL=autotools ENABLE_MRUBY=yes
-# CMake 2.8.8 or higher is required but CMake 2.8.7 is installed on Travis CI...
-#  - BUILD_TOOL=cmake
+  - BUILD_TOOL=cmake
 install:
   - curl https://raw.github.com/clear-code/cutter/master/data/travis/setup.sh | sh
   - sudo apt-get install -qq -y autotools-dev zlib1g-dev liblzo2-dev libmsgpack-dev libevent-dev libmecab-dev mecab-naist-jdic cmake