[#37] Fix dep to not prune C headers from secp256k1 lib

[placer14]

Found a solution based on https://github.com/golang/dep/issues/1725#issuecomment-374293514

This along with the ethereum override allows dep to build the vendor tree including the c headers:

index ee11806..ce1a3a1 100644
--- a/Gopkg.toml
+++ b/Gopkg.toml
@@ -116,3 +116,11 @@
 [prune]
   go-tests = true
   unused-packages = true
+
+[[override]]
+  name = "github.com/ethereum/go-ethereum"
+  version = "1.8.19"
+
+[[prune.project]]
+    name = "github.com/ethereum/go-ethereum"
+    unused-packages = false```