Simplify SoftwareSpec checksum methods

Reader methods for specific checksum types have been absent from the
Formula class for some time, so there isn't any reason to expose them in
SoftwareSpec, either.

Thus, these methods now only act as setters, and #checksum should be
used to access the constructed Checksum object.
versions: remove FIXME

I understand how this works now, and it is definitely necessary.
couchdb: depend on homebrew/versions/erlang-r15
midgard2: fix configure flag to match libgda version

Fixes #18572.
fits 0.6.2
scons 2.3.0

Closes #18566.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
RethinkDB depends on boost

Closes #18577.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
ffmpeg 1.2

Closes #18574.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
MongoDB 2.4.0

Closes #18562.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>