collected bugfixes to coincide with MetacelloPreview 1.0.0-beta.32.15 release

fixes for:

* Issue #131 cherry-pick 17b42e6898a006c93f28af2b1267d6bcec277950
* Issue #182 Need `lock` tests case involving a baseline
* Issue #225 Unclear error message when stable version not defined
* Issue #226 Still need better error reporting during MetacelloGemStonePlatform>>downloadFile:to: (curl failures) bug in process
* Issue #227 Retry error reporting in MetacelloCommonMCSpecLoader>>retryingResolvePackageSpecReferences:gofer: should be improved
* Issue #232 multiple linux users cannot share /tmp for github work
* Issue #234  `#repository: 'github://…'` does not work with branch-names containing slashes
* Issue #236 Clarify platform default repository locations
* Issue #275 downgrade of versions do not trigger `lock`