Use wp_is_writable wherever is_writable was used.
Bump required version to 3.6.0
Merge branch 'master' into wp-is-writable-function-371

Conflicts:
	functions/core.php
Merge branch 'master' into wp-is-writable-function-371

Signed-off-by: Paul de Wouters <paul@hmn.md>

Conflicts:
	backupwordpress.php
Merge pull request #371 from humanmade/wp-is-writable-function-371

Use wp_is_writable to check if backups dir is writable