Linux specific .swift-version.

Enabled linux os for travis.

.linux.swift-version

@@ -0,0 +1 @@
+3.0-PREVIEW-6

.travis.yml

@@ -1,4 +1,5 @@
 os:
+    - linux
     - osx
 language: generic
 sudo: required

CI.sh

@@ -1,4 +1,15 @@
-#!/bin/bash -ex
+#!/usr/bin/env bash
+
+set -ex
+
+OS_NAME=$TRAVIS_OS_NAME
+OS_PREFIX=""
+
+if [ $OS_NAME != "osx" ]; then
+    OS_PREFIX=".$OS_NAME"
+fi
+
+export SWIFT_VERSION=$(cat "$OS_PREFIX.swift-version")
 
 echo "Build script running as user $(whoami) in directory $(pwd)"
 
@@ -10,6 +21,8 @@ eval "$(swiftenv init -)"
 
 swiftenv version
 
+swiftenv install
+
 swift build --clean dist
 swift package fetch