Support hosts in FIPS mode

[ayohrling]

Currently, gemstash does not run on systems that are configured in FIPS mode. There are a couple spots that utilize MD5 digests that need to be replaced for valid operation. Results in the following logs when running the server and timeouts for too many connection resets in fetching gems:

md5_dgst.c(82): OpenSSL internal error, assertion failed: Digest MD5 forbidden in FIPS mode!
[2018-10-31 13:30:47 +0000] - INFO - [16128] - Worker 0 (pid: 16242) booted, phase: 0
md5_dgst.c(82): OpenSSL internal error, assertion failed: Digest MD5 forbidden in FIPS mode!
[2018-10-31 13:30:52 +0000] - INFO - [16128] - Worker 0 (pid: 16254) booted, phase: 0
md5_dgst.c(82): OpenSSL internal error, assertion failed: Digest MD5 forbidden in FIPS mode!
[2018-10-31 13:30:57 +0000] - INFO - [16128] - Worker 0 (pid: 16266) booted, phase: 0
md5_dgst.c(82): OpenSSL internal error, assertion failed: Digest MD5 forbidden in FIPS mode!
[2018-10-31 13:31:02 +0000] - INFO - [16128] - Worker 0 (pid: 16276) booted, phase: 0

[ayohrling]

Am hoping for some feedback here. I feel like this should be low-hanging fruit. PR is in, we have to work from builds off my fork to use gemstash until this is merged upstream.