Bump bcrypt from 3.1.13 to 3.1.15

Bumps bcrypt from 3.1.13 to 3.1.15.

Changelog

Sourced from bcrypt's changelog.

3.1.15 July 21 2020

• Remove GVL optimization. Apparently it breaks things [GH #230]

3.1.14 July 21 2020

• Start calibration from the minimum cost supported by the algorithm [GH #206 by @​sergey-alekseev]

Commits

• ef6efb2 bump version
• fbb89d7 Revert "unlock the gvl when calculating hashes / salts"
• a4528cf Merge pull request #227 from nathanKramer/fix/valid-hash-return-type
• bdaa2fa Fixing Java build
• 2c17728 updating lockfile
• 1b9d33c bumping version
• eb567cb Ensure valid_hash? returns a Boolean
• 2fbe541 Merge pull request #219 from sergey-alekseev/fix-v-3-1-13-changelog
• 3a1f477 Merge pull request #217 from jmartin-r7/bug/201-limit-asm
• 13ad68b fix v3.1.13 changelog entry
• Additional commits viewable in compare view