Changed MDF compression to use ROOT ZLIB instead of the default

In ROOT 6.14 the default changed from ZLIB to LZ4, and this change ensure reproducibility across versions of ROOT.

• [2018-06-13 21:56] Validation started with lhcb-gaudi-merge#527
• [2018-06-14 00:11] Validation started with lhcb-lcg-dev4#555
• [2018-06-14 00:12] Validation started with lhcb-lcg-dev3#555
• [2018-06-14 00:12] Validation started with lhcb-head#1884
• [2018-06-14 00:15] Validation started with lhcb-gaudi-head#1900
• [2018-06-14 00:17] Validation started with lhcb-tdr-test#181
• [2018-06-14 03:08] Validation started with lhcb-gaudi-head#1900

mentioned in commit e3b79c61

merged

mentioned in commit 1a492f07

mentioned in merge request !1381 (merged)