build: update dep for golang.org/x/crypto to compile under golang 1.11

Gopkg.lock

@@ -330,7 +330,7 @@
   revision = "1efa31f08b9333f1bd4882d61f9d668a70cd902e"
 
 [[projects]]
-  digest = "1:61cf25ac3ac0c8c50959666ddf732a396fbc445570be051c8b9795c8a087fde7"
+  digest = "1:270627be1fb5b0875bdbf8a96f86a551adc0aeba97fb63af3caaa21c0a39c499"
   name = "golang.org/x/crypto"
   packages = [
     "blake2b",
@@ -338,6 +338,7 @@
     "curve25519",
     "hkdf",
     "internal/chacha20",
+    "internal/subtle",
     "nacl/box",
     "nacl/secretbox",
     "pbkdf2",
@@ -349,7 +350,7 @@
     "ssh/terminal",
   ]
   pruneopts = "UT"
-  revision = "49796115aa4b964c318aad4f3084fdb41e9aa067"
+  revision = "614d502a4dac94afa3a6ce146bd1736da82514c6"
 
 [[projects]]
   digest = "1:0764abb1e99bb977d1b9f320d02859d4a737252da3e1fd233c4ae0f9522e7446"

Gopkg.toml

@@ -84,7 +84,7 @@
 
 [[constraint]]
   name = "golang.org/x/crypto"
-  revision = "49796115aa4b964c318aad4f3084fdb41e9aa067"
+  revision = "614d502a4dac94afa3a6ce146bd1736da82514c6"
 
 [[constraint]]
   name = "golang.org/x/net"