Improve Windows compatibility (#118)

- Use travis build matrix to test on Windows and macOS
- Make utility func compatible with host OS. Use filepath package to
  manipulate path strings for stripping $GOPATH from working directory.
- Delegate to go/build pkg for getting Go paths

Co-authored-by: Mike Lee <mike.lee@safeguardproperties.com>
Co-authored-by: Lucas Bremgartner <lucas@bremis.ch>

.travis.yml

@@ -6,11 +6,20 @@ branches:
   only:
     - master
 
-go:
-  - 1.11.x
-  - 1.12.x
-  - 1.13.x
-  - tip
+jobs:
+  include:
+    - os: linux
+      go: 1.11.x
+    - os: linux
+      go: 1.12.x
+    - os: linux
+      go: 1.13.x
+    - os: linux
+      go: tip
+    - os: osx
+      go: 1.13.x
+    - os: windows
+      go: 1.13.x
 
 before_install:
   - go get golang.org/x/lint/golint