Pin secp256k1 to 90faf434 due to type mismatch during compilation

[mattaudesse]

See rust-bitcoin/rust-secp256k1#100 to review commit that caused failure.

Step 11/18 : COPY src/ src/
 ---> c3e633211e8c
Step 12/18 : RUN rm ./target/release/deps/mkb_*
 ---> Running in 8a76e64a7557
Removing intermediate container 8a76e64a7557
 ---> de8f9d76c3d2
Step 13/18 : RUN cargo build --release
 ---> Running in 6d84984a5df5
   Compiling mkb v0.1.0 (/dummy_project)
error[E0308]: mismatched types
  --> src/type_sign.rs:18:29
   |
18 |     let sig_vec : Vec<u8> = secp256k1::Signature::serialize_der(&sig);
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                             |
   |                             expected struct `std::vec::Vec`, found struct `secp256k1::SerializedSignature`
   |                             help: try using a conversion method: `secp256k1::Signature::serialize_der(&sig).to_vec()`
   |
   = note: expected type `std::vec::Vec<u8>`
              found type `secp256k1::SerializedSignature`

error[E0308]: mismatched types
  --> src/type_sign.rs:18:29
   |
18 |     let sig_vec : Vec<u8> = secp256k1::Signature::serialize_der(&sig);
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                             |
   |                             expected struct `std::vec::Vec`, found struct `secp256k1::SerializedSignature`
   |                             help: try using a conversion method: `secp256k1::Signature::serialize_der(&sig).to_vec()`
   |
   = note: expected type `std::vec::Vec<u8>`
              found type `secp256k1::SerializedSignature`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `mkb`.
warning: build failed, waiting for other jobs to finish...
error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `mkb`.

To learn more, run the command again with --verbose.
ERROR: Service 'alacris_registrar_alice' failed to build: The command '/bin/sh -c cargo build --release' returned a non-zero code: 101
Makefile:33: recipe for target 'docker-build' failed
make: *** [docker-build] Error 1

[AlexKnauth]

This now successfully builds for me using docker on my computer.

[mattaudesse]

Great, thanks for trying @AlexKnauth - want to approve + merge?