handle wimba conference date parsing in ruby18/ruby19.
handle wimba conference date parsing in ruby18/ruby19.

fixes CNVS-3556

wimba passes dates as MM/DD/YYYY, but ruby 1.9 expects date
strings given to `parse` to be in DD/MM/YYYY format. this
commit specifies a date format to avoid this issue and
remain compatible with both 1.8 and 1.9 servers.

test plan:
  * create a wimba conference;
  * visit its archive;
  * verify that canvas redirects to wimba and does not
    throw a 500 error.

Change-Id: Ic822a01f950b2465a5328a0b80b8b11c3e63c99d
Reviewed-on: https://gerrit.instructure.com/17507
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Mark Ericksen <776ab05fc6b68e82d2b2c63873bf56edde131809@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>

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