Have both Linux and macOS tests on Travis.

The Swift tests will not work on Linux, because they depend on Xcode, so
we need a separate macOS run for them.

.travis.yml

@@ -1,5 +1,8 @@
 sudo: true
 language: java
+os:
+  - linux
+  - osx
 script:
   - mvn -Dparallel=methods -DthreadCount=4 install
 
@@ -8,18 +11,8 @@ jdk:
   - oraclejdk8
 
 before_install:
-  - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-  - sudo add-apt-repository ppa:fkrull/deadsnakes -y
-  - sudo add-apt-repository ppa:rwky/nodejs -y
-  - sudo apt-get update -qq
-  - sudo apt-get install -qq python3.5
-  - sudo apt-get install -qq nodejs
-  - echo "deb http://download.mono-project.com/repo/debian wheezy/snapshots/3.12.1 main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
-  - sudo apt-get install -qq mono-complete
-  - eval "$(sudo gimme 1.7.3)"
-  - go version ; go env
-  - python --version
-  - python3 --version
+  - if [[ $TRAVIS_OS_NAME == osx ]]; then ./travis/before_install-macos.sh; fi
+  - if [[ $TRAVIS_OS_NAME == linux ]]; then ./travis/before_install-linux.sh; fi
 
 env:
   - CXX=g++-5

travis/before_install-common.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -euo pipefail
+
+( go version ; go env ) || true
+python --version
+python3 --version

travis/before_install-linux.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -euo pipefail
+
+thisdir=$(dirname $(readlink -f "$0"))
+
+sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+sudo add-apt-repository ppa:fkrull/deadsnakes -y
+sudo add-apt-repository ppa:rwky/nodejs -y
+sudo apt-get update -qq
+sudo apt-get install -qq python3.5
+sudo apt-get install -qq nodejs
+echo "deb http://download.mono-project.com/repo/debian wheezy/snapshots/3.12.1 main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
+sudo apt-get install -qq mono-complete
+eval "$(sudo gimme 1.7.3)"
+
+"$thisdir/before_install-common.sh"

travis/before_install-macos.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -euo pipefail
+
+thisdir=$(dirname $(readlink -f "$0"))
+
+"$thisdir/before_install-common.sh"