Automatically agree to licenses in DMG files

Otherwise installation just fails. :-(

See https://github.com/timshadel/puppet-vitamin-r for a manifest
file referencing such a DMG file.

Here's the failure output of that DMG (before this patch):

    Debug: Executing '/usr/bin/curl -o /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/d20130604-54791-kkvyrl/Vitamin-R -C - -k -L -s --url http://www.publicspace.net/download/Vitamin.dmg'
    Debug: Success: curl transfered [Vitamin-R]
    Debug: Executing '/usr/bin/hdiutil mount -plist -nobrowse -readonly -mountrandom /tmp /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/d20130604-54791-kkvyrl/Vitamin-R'
    Error: Execution of '/usr/bin/hdiutil mount -plist -nobrowse -readonly -mountrandom /tmp /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/d20130604-54791-kkvyrl/Vitamin-R' returned 1: 
    Disclaimer of Warranty

    This software is sold "as is" and without warranties as to performance
    or merchantability or any other warranties whether expressed or
    implied. Because of the various hardware and software environments
    into which this program may be put, no warranty of fitness for a
    particular purpose is offered. Good data processing procedure
    dictates that any program be throughly tested with non-critical
    data before relying on it. The user must assume the entire risk of
    using the program. Any liability of the seller or author of the
    program will be limited exclusively to product replacement or refund
    of purchase price.
    hdiutil: mount canceled

    Error: /Stage[main]/Vitamin_r/Package[Vitamin-R]/ensure: change from absent to present failed: Execution of '/usr/bin/hdiutil mount -plist -nobrowse -readonly -mountrandom /tmp /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/d20130604-54791-kkvyrl/Vitamin-R' returned 1: 
    Disclaimer of Warranty

    This software is sold "as is" and without warranties as to performance
    or merchantability or any other warranties whether expressed or
    implied. Because of the various hardware and software environments
    into which this program may be put, no warranty of fitness for a
    particular purpose is offered. Good data processing procedure
    dictates that any program be throughly tested with non-critical
    data before relying on it. The user must assume the entire risk of
    using the program. Any liability of the seller or author of the
    program will be limited exclusively to product replacement or refund
    of purchase price.
    hdiutil: mount canceled

And again, after the patch:

    Debug: Executing '/usr/bin/curl -o /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/d20130604-58407-1h1fssy/Vitamin-R -C - -k -L -s --url http://www.publicspace.net/download/Vitamin.dmg'
    Debug: Success: curl transfered [Vitamin-R]
    Debug: Executing '/usr/bin/yes | /usr/bin/hdiutil mount -plist -nobrowse -readonly -mountrandom /tmp /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/d20130604-58407-1h1fssy/Vitamin-R 2> /dev/null'
    Debug: Executing '/usr/bin/ditto --rsrc /private/tmp/dmg.OiSb9K/Vitamin-R 2.app /Applications/Vitamin-R 2.app'
    Debug: Executing '/usr/bin/hdiutil eject /private/tmp/dmg.OiSb9K'
    Notice: /Stage[main]/Vitamin_r/Package[Vitamin-R]/ensure: created
    Debug: /Stage[main]/Vitamin_r/Package[Vitamin-R]: The container Class[Vitamin_r] will propagate my refresh event