Remove redundant if ...; err != nil checks

[davidklassen]

Fixes golint warnings:

$ golint ./...
atomicfile.go:41:2: redundant if ...; err != nil check, just return error instead.
atomicfile.go:55:2: redundant if ...; err != nil check, just return error instead.

[davidklassen]

It looks like vet command is no longer available under golang.org/x/tools/cmd package (see https://golang.org/doc/go1.2#go_tools_godoc). May be it makes sense to update go version in .travis.yml?

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need the corporate CLA signed.

If you have received this in error or have any questions, please contact us at cla@fb.com. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!