[YSM] fix array key regex

[jrushlow]

This fixes an issue if an existing array has a : within the value, YamlSourceManipulator could mistake part of the value for the array key when determining position.

E.g.

config:
    - '80:80'

YSM would pick the 0: within 80:80 as the array key.

[jrushlow]

This should fix #777. But does not handle passing a literal "80:80" as a new array value and getting '80:80' in return. PR #782 is a WIP for this "issue" with an unknown future.

[weaverryan]

Thanks for hunting this down Jesse!