change back checksum for libdap 3.19.1

[boegel]

This reverts commit 19d037e (PR #5473), since apparently the original tarball was re-instated (on Dec 28th 2017, if the timestamp in https://www.opendap.org/pub/source/ can be trusted).

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in this PR)
node2581.golett.os - Linux centos linux 7.4.1708, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz, Python 2.7.5
See https://gist.github.com/3789f80cd1ce2d072203dc6242db6ad2 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in this PR)
node2082.delcatty.os - Linux centos linux 7.4.1708, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.7.5
See https://gist.github.com/4442935f0618c3367ce79bca7d1b0056 for a full test report.

[vanzod]

Test report by @vanzod
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in this PR)
cermis - Linux debian 9.4, Intel(R) Xeon(R) CPU E5620 @ 2.40GHz, Python 2.7.13
See https://gist.github.com/c6a58f944e6d8d1e0c659cd48824c1b1 for a full test report.

[vanzod]

Going in, thanks @boegel!