Switch to go modules for dependency management.

Added support for dependency management using go modules.
Note that Go version 1.11+ is required if using go modules for
dependency management.

Updated the gitlab CI yaml file to use go modules instead of copying
everything to GOPATH. Ran go mod download before running build script to
download all dependencies into cache.

.gitlab-ci.yml

@@ -1,18 +1,18 @@
-image: golang:1.11
+image: golang:1.12
 
 cache:
   paths:
-    - /apt-cache
-    - /go/src/github.com
-    - /go/src/golang.org
-    - /go/src/google.golang.org
-    - /go/src/gopkg.in
+    - /go/pkg/mod/github.com
+    - /go/pkg/mod/golang.org
+    - /go/pkg/mod/google.golang.org
 
 before_script:
-  - mkdir -p /go/src/gitlab.com/spdf /go/src/_/builds
-  - cp -r $CI_PROJECT_DIR /go/src/gitlab.com/spdf/elektron
-  - ln -s /go/src/gitlab.com/spdf /go/src/_/builds/elektron
+  - export GOPATH="/go"
+  - mkdir -p $GOPATH/pkg/mod github.com $GOPATH/pkg/mod/golang.org $GOPATH/pkg/mod/google.golang.org
+  - cp -r $CI_PROJECT_DIR /elektron
+  - cd /elektron
+  - go mod download
 
 build:
   script:
-    - go build -o elektron /go/src/gitlab.com/spdf/elektron
+    - go build -v -o elektron