Turn Linux build into macOS build on Travis CI

For #27

.travis.yml

@@ -1,18 +1,21 @@
 git:
   depth: 1
 
-language: go
+os: osx
 
-go:
-  - "1.12"
+addons:
+  homebrew:
+    packages:
+    - [email protected] # Currently the default Go version on the macOS build server is 1.10
+    - upx
+    update: true
 
 env:
   - GO111MODULE=on
 
 before_install:
   - go version
   - go env
-  - sudo apt-get install -y upx-ucl
 
 script:
   - go build -v
@@ -21,6 +24,8 @@ script:
   - ./serve -h
   - ./serve -v
   - ./serve -t
-  - build/artifacts/serve_v*_Linux_x64 -h
-  - build/artifacts/serve_v*_Linux_x64 -v
-  - build/artifacts/serve_v*_Linux_x64 -t
+  - VERSION=$(<VERSION)
+  # Can't use "serve_v*_" for globbing on macOS by default
+  - build/artifacts/serve_v${VERSION}_macOS_x64 -h
+  - build/artifacts/serve_v${VERSION}_macOS_x64 -v
+  - build/artifacts/serve_v${VERSION}_macOS_x64 -t