Added sort to guarantee ordering of executions

When Camus is deleting old executions and finding the latest, the
ordering of executions was not guaranteed. (i.e. Camus may find a
previous execution that isn't the latest). This has been an issue when
testing Camus using the local file system or under certain Hadoop
distributions. The listing of execution files is now sorted into 'last
modified' order.
Use lexicographical order

Use lexicographical order  instead of 'last modified' order for
determining the latest executions
Merge pull request #48 from jfg9/master

Added sort to guarantee ordering of executions