Fix lmdb install for travis

[shelhamer]

switch to git mirror

• risky since lmdb might change upstream
• download the latest tag... which looks safe enough given tags from 2013 are still present
• safe b.c. the static download urls go stale

[jeffdonahue]

risky since lmdb might change upstream

How about checking out the latest tagged release? (LMDB_0.9.14 it looks like?)

edit: or we could just change the URL to https://github.com/LMDB/lmdb/archive/mdb.RE/0.9.zip -- I think the GitHub mirror should be safe? (As I recall last time I checked (the last time the URL went down) there was no official github mirror.) Cloning at the release tag with --depth 1 should download a pretty comparable amount though.

[shelhamer]

Amended to just download the tarball... let's see if this works.

[ronghanghu]

The current link ftp://ftp.openldap.org/pub/OpenLDAP/openldap-release/openldap-2.4.39.tgz seems to have resurrected a few minutes ago, so my PR #2527 now passes Travis. But indeed a stable download link is needed and I agree lmdb github release should be a stable one.