v0.5.4

What's Changed

• Add mutable accessors for some important Vec/HashMap fields by @cgwalters in #87
• Use buffered I/O for JSON by default by @cgwalters in #89
• impl Default for Os and Arch, use in ImageConfiguration::default() by @cgwalters in #90
• image: Fix powerpc, add a test case for arch mapping by @cgwalters in #91

Full Changelog: v0.5.3...v0.5.4