.travis.yml -> swift 5 and xcode 10.2

Author: Mike Pollard

.travis.yml

@@ -1,11 +1,14 @@
 notifications:
   slack: zewo:VjyVCCQvTOw9yrbzQysZezD1
 language: generic
+env:
+  global:
+  - SWIFT_VERSION=5.0
 matrix:
   include:
     - os: osx
       env: JOB=SwiftPM_OSX
-      osx_image: xcode10.1
+      osx_image: xcode10.2
     - os: linux
       env: JOB=SwiftPM_linux
       dist: trusty