Upgraded to latest Rust

Closes https://github.com/PistonDevelopers/image/issues/107

Travis CI might fail if it is not using latest Rust nightly
Merge pull request #128 from bvssvni/master

Upgraded to latest Rust
Renamed to suffix `_mut`

Closes https://github.com/PistonDevelopers/image/issues/129
Update for rustc
Merge pull request #131 from tomaka/master

Update for rustc
Merge pull request #130 from bvssvni/master

Renamed to suffix `_mut`
Merge commit 'b5b1c81474fed34206f6b27968c39cf1abb6c750'