Update remaining Rust dependencies

Cargo.lock

@@ -465,9 +465,9 @@ dependencies = [
 
 [[package]]
 name = "ffi_helpers"
-version = "0.1.0"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "545c5db858a1f8749e81b22cb9e10a5412c9775441a53c4bd1d1f28531edc843"
+checksum = "36a21261040ffbc3999698b39170d6e951f2823dd9ad7b50469309156060a50c"
 dependencies = [
  "failure",
  "failure_derive",
@@ -565,12 +565,6 @@ dependencies = [
  "libc",
 ]
 
-[[package]]
-name = "hex"
-version = "0.3.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "805026a5d0141ffc30abb3be3173848ad46a1b1664fe632428479619a3644d77"
-
 [[package]]
 name = "hex"
 version = "0.4.2"
@@ -627,7 +621,7 @@ dependencies = [
  "cbindgen",
  "failure",
  "ffi_helpers",
- "hex 0.3.2",
+ "hex",
  "zcash_client_backend",
  "zcash_client_sqlite",
  "zcash_primitives",
@@ -1302,7 +1296,7 @@ dependencies = [
  "bs58",
  "ff",
  "group",
- "hex 0.4.2",
+ "hex",
  "jubjub",
  "protobuf",
  "protobuf-codegen-pure",
@@ -1346,7 +1340,7 @@ dependencies = [
  "ff",
  "fpe",
  "group",
- "hex 0.4.2",
+ "hex",
  "jubjub",
  "lazy_static",
  "log",

Cargo.toml

@@ -9,8 +9,8 @@ build = "rust/build.rs"
 
 [dependencies]
 failure = "0.1"
-ffi_helpers = "0.1"
-hex = "0.3"
+ffi_helpers = "0.2"
+hex = "0.4"
 zcash_client_backend = "0.4"
 zcash_client_sqlite = "0.2"
 zcash_primitives = "0.4"