chore(deps): bump secp256k1 from 0.15.1 to 0.15.5

[dependabot-preview]

Bumps secp256k1 from 0.15.1 to 0.15.5.

Changelog

Sourced from secp256k1's changelog.

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
  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
  PR 2.
  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

... (truncated)

Commits

• f5c8a00 Merge pull request #159 from rust-bitcoin/2019-09-0155
• 0c901db bump version to 0.15.5
• 0e5e314 Merge pull request #158 from rust-bitcoin/revert-153-2019-08-rand-std-feature
• bbab1c2 Revert "Add a rand-std feature"
• 4bcb404 Merge #156: Pin version of build dependency cc to <1.0.42
• 845bff2 Bump version to 0.15.4
• 07ef3a1 Pin version of build dependency cc to <1.0.42
• 4e69dcc Merge pull request #153 from elichai/2019-08-rand-std-feature
• b59f4ff Add a rand-std feature
• eddfe03 Merge pull request #151 from elichai/2019-08-Cptr-null
• Additional commits viewable in compare view

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[doitian]

@nervos-bot-user give me five

bors r+

[nervos-bot]

🚢 requested by @doitian in #1580 (comment)

[bors]

Merge conflict (retrying...)

[bors]

Canceled

[zhangsoledad]

bors r+

[bors]

Build failed

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[zhangsoledad]

@dependabot rebase

[zhangsoledad]

bors r+

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