Update when to version 1.7.1

This removes the need for us to hack out the freezing of promises: they removed it upstream (see cujojs/when#61 ) until V8 fixes the major performance problem with frozen objects.
Merge pull request #424 from AnalyticalGraphicsInc/upgrade_when

Update when to version 1.7.1
Fix issue #425.

Do not allow a TMS imagery provider's extent to be outside the extent
alowed by the tiling scheme, no matter what the tilemapresource.xml file
says.
Merge pull request #430 from AnalyticalGraphicsInc/issue425

Fix issue #425.
Merge branch 'master' into imageryTutorialExample