(#460) Extend license mapping for common SPDX license names (#509)

Fixes #460 
Signed-off-by: Christopher Angelo Phillips <christopher.phillips@anchore.com>

internal/spdxlicense/license_list_test.go

@@ -10,5 +10,6 @@ func TestLicenceListIDs(t *testing.T) {
 	// do a sanity check on the generated data
 	assert.Equal(t, "0BSD", licenseIDs["0bsd"])
 	assert.Equal(t, "ZPL-2.1", licenseIDs["zpl-2.1"])
+	assert.Equal(t, "GPL-2.0", licenseIDs["gpl-2"])
 	assert.NotEmpty(t, Version)
 }