update source for libsodium to 1.0.15

compiling fails with older version than 1.0.13 and libsodium needs #
to be compiled from the scratch for ubuntu 14.04 and 16.04

solves BTCGPU#43
see also BTCGPU#53

depends/packages/libsodium.mk

@@ -7,7 +7,7 @@ package=libsodium
 $(package)_version=1.0.15
 $(package)_download_path=https://download.libsodium.org/libsodium/releases
 $(package)_file_name=$(package)-$($(package)_version).tar.gz
-$(package)_sha256_hash=RWQf6LRCGA9i53mlYecO4IzT51TGPpvWucNSCh1CBM0QTaLn73Y7GFO3
+$(package)_sha256_hash=FB6A9E879A2F674592E4328C5D9F79F082405EE4BB05CB6E679B90AFE9E178F4
 $(package)_dependencies=
 $(package)_config_opts=
 $(package)_config_opts_linux=--with-pic