Revert "Installing pexif from untrusted source" (fixes #270)

This reverts commit adbb51c75bbb0d5c1268b84413e99b7f33dbe572.

pexif 0.13 now installs from PyPI, see bennoleslie/pexif#3
Merge pull request #282 from scorphus/pexif

Revert "Installing pexif from untrusted source" (fixes #270)