Update base64 requirement from 0.20 to 0.21

Updates the requirements on [base64](https://github.com/marshallpierce/rust-base64) to permit the latest version.
- [Release notes](https://github.com/marshallpierce/rust-base64/releases)
- [Changelog](https://github.com/marshallpierce/rust-base64/blob/master/RELEASE-NOTES.md)
- [Commits](marshallpierce/rust-base64@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: base64
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

postgres-protocol/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/sfackler/rust-postgres"
 readme = "../README.md"
 
 [dependencies]
-base64 = "0.20"
+base64 = "0.21"
 byteorder = "1.0"
 bytes = "1.0"
 fallible-iterator = "0.2"