Add a travis CI config file to run a Mac build

I mirror the repo at https://github.com/acts-trk/acts-core. This can be used to trigger CI builds when GitHub receives a push from GitLab. This will not be able to restrict merge requests, but at least it will give us an automatic insight into whether the Mac build works at any point.

Currently one of the tests actually fails (see #605 (closed)). I still haven't been able to reproduce this locally, so wasn't able to fix it so far.

In addition, Travis CI also runs a build without DD4hep and ROOT on Ubuntu, and another one with all plugins on LCG94 through a docker container, and an explicit CVMFS install (I thought I'd add this while I'm at it)

.travis.yml

+language: cpp
+
+matrix:
+  include:
+    - os: osx
+      osx_image: xcode10.2
+      before_install:
+        - brew install eigen
+    - os: linux
+      addons:
+        apt:
+          update: true
+          sources:
+            - sourceline: 'ppa:mhier/libboost-latest'
+            - ubuntu-toolchain-r-test
+          packages:
+            - libeigen3-dev
+            - boost1.67
+            - cmake
+            - g++-7
+      env: CC=gcc-7 CXX=g++-7
+    - os: linux
+      sudo: required
+      before_install:
+        - wget --no-check-certificate https://ecsft.cern.ch/dist/cvmfs/cvmfs-release/cvmfs-release-latest_all.deb
+        - sudo dpkg -i cvmfs-release-latest_all.deb
+        - sudo apt-get update
+        - sudo apt-get install cvmfs cvmfs-config-default
+        - rm -f cvmfs-release-latest_all.deb
+        - wget --no-check-certificate https://lcd-data.web.cern.ch/lcd-data/CernVM/default.local
+        - sudo mkdir -p /etc/cvmfs
+        - sudo mv default.local /etc/cvmfs/default.local
+        - sudo /etc/init.d/autofs stop
+        - sudo cvmfs_config setup
+        - sudo mkdir -p /cvmfs/sft.cern.ch
+        - sudo mount -t cvmfs sft.cern.ch /cvmfs/sft.cern.ch
+        - ls /cvmfs/sft.cern.ch
+      services:
+        - docker
+      script:
+        - docker run -ti --name CI_container -v ${PWD}:/src -w /src -v"/cvmfs:/cvmfs:ro" -d gitlab-registry.cern.ch/acts/machines/cc7 /bin/bash
+        - docker exec -ti CI_container /bin/bash -c "source CI/setup_lcg94.sh && mkdir build && cd build && cmake .. -DACTS_BUILD_EXAMPLES=on -DACTS_BUILD_TESTS=on -DACTS_BUILD_INTEGRATION_TESTS=on -DACTS_BUILD_DIGITIZATION_PLUGIN=on -DACTS_BUILD_IDENTIFICATION_PLUGIN=on -DACTS_BUILD_JSON_PLUGIN=on -DACTS_BUILD_LEGACY=on -DACTS_BUILD_DD4HEP_PLUGIN=on -DACTS_BUILD_TGEO_PLUGIN=on && cmake --build . -- -j$(nproc) && cmake --build . -- test"
+
+script:
+  - mkdir build && cd build
+  - cmake .. -DCMAKE_CXX_FLAGS="-std=c++17" -DACTS_BUILD_EXAMPLES=on -DACTS_BUILD_TESTS=on -DACTS_BUILD_INTEGRATION_TESTS=on -DACTS_BUILD_DIGITIZATION_PLUGIN=on -DACTS_BUILD_IDENTIFICATION_PLUGIN=on -DACTS_BUILD_JSON_PLUGIN=on -DACTS_BUILD_LEGACY=on
+  - cmake --build . -- -j$(nproc)
+  - cmake --build . -- test
+
+
+
+