Skip attribute events in watcher. Fixes issue #460

On filesystems with atime enabled (like OSX) an infinite loop can be
triggered. git describe --dirty which is used to set APP_VERSION
changes atime of project files and a rebuild is triggered which tries
to set the APP_VERSION. More details in the issue.
Merge pull request #475 from ottob/issue460

Add attribute event ignoring in watcher. Fixes issue #460