mapbox#89: updated Travis CI config to use Swift 5.0

Author: Victor Kononov

.travis.yml

@@ -4,7 +4,7 @@ language: generic
 sudo: required
 dist: trusty
 env:
-  - SWIFT_VERSION=4.1
+  - SWIFT_VERSION=5.0
 install:
   - eval "$(curl -sL https://swiftenv.fuller.li/install.sh)"
 script: