Feature/nested modules (#6)

* add support for nested go modules

* update dependencies

pkg/vcs/git_test.go

@@ -52,6 +52,20 @@ func TestGit(t *testing.T) {
 			Timestamp: "2018-09-21",
 			Checksum:  "YIusKhLwlEKiuwowBFdEElpP0hIGDOCaSnQaMufLB00=",
 		},
+		{
+			// Repository that contains go.mod
+			Module:    "bitbucket.org/gomodproxytest/nested",
+			Tag:       "v1.0.0",
+			Timestamp: "2018-10-18",
+			Checksum:  "rFPd/yPMmhjy4FdmLTySAHVcjHmZh/XP3xNrHKLiFss=",
+		},
+		{
+			// Submodule that contains its own go.mod and tag
+			Module:    "bitbucket.org/gomodproxytest/nested/child",
+			Tag:       "v1.0.0",
+			Timestamp: "2018-10-18",
+			Checksum:  "RN9U68h7BnmrsLP24VM37/Zeyb+21R8KmGZtxTUBd74=",
+		},
 		{
 			// Just a frequently used module from github
 			Module:    "github.com/pkg/errors",