Build broken for modern DMD compiler

```
dcrypt 0.0.7: building configuration "libdcrypt"...
../../.dub/packages/dcrypt-0.0.7/dcrypt/source/dcrypt/crypto/bufferedblockcipher.d(52,41): Error: Built-in hex string literals are obsolete, use std.conv.hexString!"2b7e151628aed2a6abf7158809cf4f3c" instead.
```