Updated the fetching method of GOPATH. Code restructuring

Updated the fetching method for GOPATH
Updated the test case to correctly test the GOPATH

notice.go

@@ -2,9 +2,9 @@ package gobrake
 
 import (
 	"fmt"
+	"go/build"
 	"net/http"
 	"os"
-	"path/filepath"
 	"runtime"
 	"strings"
 	"sync"
@@ -45,23 +45,10 @@ func getDefaultContext() map[string]interface{} {
 }
 
 func gopath() string {
-	path := os.Getenv("GOPATH")
-	if path != "" {
+	if path, ok := os.LookupEnv("GOPATH"); ok {
 		return path
 	}
-
-	path, ok := os.LookupEnv("HOME")
-	if ok {
-		return filepath.Join(path, "go")
-	}
-
-	return ""
-}
-
-type Error struct {
-	Type      string       `json:"type"`
-	Message   string       `json:"message"`
-	Backtrace []StackFrame `json:"backtrace"`
+	return build.Default.GOPATH
 }
 
 type StackFrame struct {
@@ -71,6 +58,12 @@ type StackFrame struct {
 	Code map[int]string `json:"code,omitempty"`
 }
 
+type Error struct {
+	Type      string       `json:"type"`
+	Message   string       `json:"message"`
+	Backtrace []StackFrame `json:"backtrace"`
+}
+
 type Notice struct {
 	Id    string `json:"-"` // id returned by SendNotice
 	Error error  `json:"-"` // error returned by SendNotice

notifier_test.go

@@ -6,6 +6,7 @@ import (
 	"crypto/rand"
 	"encoding/json"
 	"errors"
+	"go/build"
 	"io/ioutil"
 	"log"
 	"net/http"
@@ -343,6 +344,9 @@ var _ = Describe("Notifier", func() {
 
 		hostname, _ := os.Hostname()
 		gopath := os.Getenv("GOPATH")
+		if gopath == "" {
+			gopath = build.Default.GOPATH
+		}
 		wd, _ := os.Getwd()
 
 		Expect(sentNotice.Context["language"]).To(Equal(runtime.Version()))