Switch to sort Interface to be backward compatible with older go versions

cmd/gas/main.go

@@ -236,7 +236,7 @@ func main() {
 
 	// Sort the issue by severity
 	if *flagSortIssues {
-		sort.Slice(issues, func(i, j int) bool { return (issues[i].Severity > issues[j].Severity) })
+		sortIssues(issues)
 	}
 
 	// Create output report

cmd/gas/sort_issues.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"sort"
+
+	"github.com/GoASTScanner/gas"
+)
+
+type sortBySeverity []*gas.Issue
+
+func (s sortBySeverity) Len() int { return len(s) }
+
+func (s sortBySeverity) Less(i, j int) bool { return s[i].Severity > s[i].Severity }
+
+func (s sortBySeverity) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+// sortIssues sorts the issues by severity in descending order
+func sortIssues(issues []*gas.Issue) {
+	sort.Sort(sortBySeverity(issues))
+}