try pre-installed gecode

.travis.yml

@@ -24,11 +24,13 @@ addons:
   apt:
     packages:
       - libgmp-dev
+      - libgecode-dev
 
 before_install:
 # download and build gecode
-- travis_retry curl -L https://github.com/Gecode/gecode/archive/release-6.0.0.tar.gz | tar xz
-- cd gecode-release-6.0.0 && ./configure --prefix=/opt/gecode/gecode-6.0.0 && make && make install
+# NO: the build takes too long, and is cut off by travis
+# - travis_retry curl -L https://github.com/Gecode/gecode/archive/release-6.0.0.tar.gz | tar xz
+# - cd gecode-release-6.0.0 && ./configure --prefix=/opt/gecode/gecode-6.0.0 && make && make install
 # Download and unpack the stack executable
 - mkdir -p ~/.local/bin
 - export PATH=$HOME/.local/bin:$PATH