Merge #1580

1580: chore(deps): bump secp256k1 from 0.15.1 to 0.15.5 r=zhangsoledad a=dependabot-preview[bot]

Bumps [secp256k1](https://github.com/rust-bitcoin/rust-secp256k1) from 0.15.1 to 0.15.5.
<details>
<summary>Changelog</summary>

*Sourced from [secp256k1's changelog](https://github.com/rust-bitcoin/rust-secp256k1/blob/master/CHANGELOG.md).*

> 
> # 0.15.4 - 2019-09-06
> 
> - Add `rand-std` feature.
> - Pin the cc build-dep version to `< 1.0.42` to remain
>   compatible with rustc 1.22.0.
> - Changed all `as_*ptr()` to a new safer `CPtr` trait
> 
> # 0.15.2 - 2019-08-08
> 
> - Add feature `lowmemory` that reduces the EC mult window size to require
>   significantly less memory for the validation context (~680B instead of
>   ~520kB), at the cost of slower validation. It does not affect the speed of
>   signing, nor the size of the signing context.
> 
> # 0.15.0 - 2019-07-25
> 
> * Implement hex human-readable serde for PublicKey
> * Implement fmt::LowerHex for SecretKey and PublicKey
> * Relax `cc` dependency requirements
> * Add links manifest key to prevent cross-version linkage
> 
> # 0.14.1 - 2019-07-14
> 
> * Implemented FFI functions: `secp256k1_context_create` and `secp256k1_context_destroy` in rust.
> 
> # 0.14.0 - 2019-07-08
> 
> * [Feature-gate endormorphism optimization](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/pull/120)
>   because of a lack of clarity with respect to patents
> * Got full no-std support including eliminating all use of libc in C bindings.
>   [PR 1](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/pull/115)
>   [PR 2](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/pull/125).
>   This library should be usable in bare-metal environments and with rust-wasm.
>   Thanks to Elichai Turkel for driving this forward!
> 
> # 0.13.0 - 2019-05-21
> 
> * Update minimum supported rust compiler 1.22.
> * Replace `serialize_der` function with `SerializedSignature` struct.
> * Allow building without a standard library (`no_std`). `std` feature is on by default.
> * Add human readable serialization to `Signatures` and `SecretKeys`.
> * Stop displaying 0 bytes if a `Signature` is less than 72 bytes.
> * Only compile recovery module if feature `recovery` is set (non-default).
> * Update `rand` dependency from 0.4 to 0.6 and add `rand_core` 0.4 dependency.
> * Relax `cc` dependency requirements.
> 
> # 0.12.2 - 2019-01-18
> 
> * Fuzzer bug fix
></tr></table> ... (truncated)
</details>
<details>
<summary>Commits</summary>

- [`f5c8a00`](rust-bitcoin/rust-secp256k1@f5c8a00) Merge pull request [#159](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/issues/159) from rust-bitcoin/2019-09-0155
- [`0c901db`](rust-bitcoin/rust-secp256k1@0c901db) bump version to 0.15.5
- [`0e5e314`](rust-bitcoin/rust-secp256k1@0e5e314) Merge pull request [#158](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/issues/158) from rust-bitcoin/revert-153-2019-08-rand-std-feature
- [`bbab1c2`](rust-bitcoin/rust-secp256k1@bbab1c2) Revert "Add a rand-std feature"
- [`4bcb404`](rust-bitcoin/rust-secp256k1@4bcb404) Merge [#156](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/issues/156): Pin version of build dependency cc to <1.0.42
- [`845bff2`](rust-bitcoin/rust-secp256k1@845bff2) Bump version to 0.15.4
- [`07ef3a1`](rust-bitcoin/rust-secp256k1@07ef3a1) Pin version of build dependency cc to <1.0.42
- [`4e69dcc`](rust-bitcoin/rust-secp256k1@4e69dcc) Merge pull request [#153](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/issues/153) from elichai/2019-08-rand-std-feature
- [`b59f4ff`](rust-bitcoin/rust-secp256k1@b59f4ff) Add a rand-std feature
- [`eddfe03`](rust-bitcoin/rust-secp256k1@eddfe03) Merge pull request [#151](https://github-redirect.dependabot.com/rust-bitcoin/rust-secp256k1/issues/151) from elichai/2019-08-Cptr-null
- Additional commits viewable in [compare view](rust-bitcoin/rust-secp256k1@0.15.1...0.15.5)
</details>
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