Release 0.6 (petgraph#452)

* Version 0.6.0

* Update changelog with release date

Version 0.5.1

Prepare for 0.5.0 release

(cargo-release) version 0.4.13

(cargo-release) version 0.4.11

(cargo-release) version 0.4.10

(cargo-release) version 0.4.9

(cargo-release) version 0.4.8

(cargo-release) version 0.4.7

(cargo-release) version 0.4.6