Update base64 requirement from 0.11 to 0.12

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@v0.11.0...v0.12.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

postgres-protocol/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/sfackler/rust-postgres"
 readme = "../README.md"
 
 [dependencies]
-base64 = "0.11"
+base64 = "0.12"
 byteorder = "1.0"
 bytes = "0.5"
 fallible-iterator = "0.2"