adding travis configuration for swift under linux

.travis.yml

@@ -36,6 +36,11 @@ matrix:
       compiler: clang
       osx_image: xcode8.1
       env: TARGET=swift
+    - os: linux
+      compiler: clang
+      env:
+        - TARGET=swift
+        - SWIFT_VERSION=swift-3.1.1
     - os: osx
       osx_image: xcode8.2
       env: TARGET=dotnet

.travis/before-install-linux-swift.sh

@@ -0,0 +1,19 @@
+set -euo pipefail
+
+# download swift
+mkdir swift
+curl https://swift.org/builds/$SWIFT_VERSION-release/ubuntu1404/$SWIFT_VERSION-RELEASE/$SWIFT_VERSION-RELEASE-ubuntu14.04.tar.gz -s | tar xz -C swift &> /dev/null
+
+# install dependencies
+sudo apt-get install clang libicu52
+
+# update libstdc++6, travis uses some old version
+echo "\n" | sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+sudo apt-get update
+sudo apt-get install gcc-4.9
+sudo apt-get install libstdc++6
+
+# check swift
+export PATH=$(pwd)/swift/$SWIFT_VERSION-RELEASE-ubuntu14.04/usr/bin:$PATH
+swift --version
+swift build --version