Update base64 requirement from 0.12.0 to 0.13.0

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](https://github.com/marshallpierce/rust-base64/compare/v0.12.0...v0.13.0)

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

Cargo.toml

@@ -18,7 +18,7 @@ tls = ["native-tls"]
 tls-vendored = ["native-tls", "native-tls/vendored"]
 
 [dependencies]
-base64 = "0.12.0"
+base64 = "0.13.0"
 byteorder = "1.3.2"
 bytes = "0.5"
 http = "0.2"