.travis.yml: Show core dump when it happens

From http://jsteemann.github.io/blog/2014/10/30/getting-core-dumps-of-failed-travisci-builds/

.travis.yml

@@ -23,6 +23,7 @@ before_install:
   # Install cask
   - curl -fsSL https://raw.githubusercontent.com/cask/cask/master/go | python
 install:
+  - sudo apt-get install -y gdb
   - sudo apt-get install -y python3 python3-pip &> /dev/null
   - sudo pip3 install --upgrade setuptools pip &> /dev/null
   # --ignore-installed six since jupyter tries to upgrade it, but it can't be
@@ -40,6 +41,11 @@ install:
   - jupyter --runtime-dir
   - jupyter --paths
   - jupyter kernelspec list
+before_script:
+  - ulimit -c unlimited -S
+after_failure:
+  - COREFILE=$(find . -maxdepth 1 -name "core*" | head -n 1)
+  - if [[ -f "$COREFILE" ]]; then gdb -c "$COREFILE" example -ex "thread apply all bt" -ex "set pagination 0" -batch; fi
 script:
   - cd $TRAVIS_BUILD_DIR
   - make compile