Update png requirement from ^0.14 to ^0.15 in /examples

Updates the requirements on [png](https://github.com/image-rs/image-png) to permit the latest version.
- [Release notes](https://github.com/image-rs/image-png/releases)
- [Commits](image-rs/image-png@v0.14.1...v0.15.0)

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

Author: dependabot-preview[bot]

examples/aobench/Cargo.toml

@@ -16,7 +16,7 @@ path = "src/lib.rs"
 [dependencies]
 structopt = "^0.2"
 failure = "^0.1"
-png = "^0.14"
+png = "^0.15"
 packed_simd = { path = "../.." }
 rayon = "^1.0"
 time = "^0.1"