Make the certificate data read-only. (#7)

root_certificates.cc

diff --git a/root_certificates.cc b/root_certificates.cc
index 5a6485d..afebe71 100644
--- a/root_certificates.cc
+++ b/root_certificates.cc
@@ -14,7 +14,7 @@
 namespace dart {
 namespace bin {
 
-unsigned char root_certificates_pem_[] = {
+const unsigned char root_certificates_pem_[] = {
   0x0a, 0x2d, 0x2d, 0x2d, 0x2d, 0x2d, 0x42, 0x45, 0x47, 0x49, 0x4e, 0x20,
   0x43, 0x45, 0x52, 0x54, 0x49, 0x46, 0x49, 0x43, 0x41, 0x54, 0x45, 0x2d,
   0x2d, 0x2d, 0x2d, 0x2d, 0x0a, 0x4d, 0x49, 0x49, 0x44, 0x64, 0x54, 0x43,