Fixing serde overflow error (#1977)

[debris]

No description provided.

[debris]

updated also secp library, cause travis was failing to compile it

[rphmeier]

rust-beta build is failing, looks like a type inference error of some kind.

[debris]

I fixed the issues. Now rustc beta failed 3 times in a row with

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
The build has been terminated

But I believe that it is still worth merging since current stable has issues with compiling with rustc stable