depends: fix typo in source downloading. Regression from c897b1e732.

Some sources are renamed after download, since the filenames don't play nice
with (for example) gitian. This fixes the rename.

Needed for OSX build as it renames a file.
travis: add osx build
Merge pull request #4843

81061ff travis: add osx build (Cory Fields)
e4cf9f8 depends: fix typo in source downloading. Regression from c897b1e732. (Cory Fields)