Tweak tropism weight in king danger

There was a simplification attempt last week for the tropism
term in king danger, which passed STC but failed LTC. This
was an indirect sign that maybe the tropism factor was sightly
untuned in current master, so we tried to change it from 1/4
to 5/16.

STC:
LLR: 2.95 (-2.94,2.94) [0.00,4.00]
Total: 28098 W: 6264 L: 5990 D: 15844
http://tests.stockfishchess.org/tests/view/5c518db60ebc593af5d4e306

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 103709 W: 17387 L: 16923 D: 69399
http://tests.stockfishchess.org/tests/view/5c52a5510ebc592fc7baea8b

Bench: 4016000

src/evaluate.cpp

@@ -483,7 +483,7 @@ namespace {
                  +  69 * kingAttacksCount[Them]
                  + 185 * popcount(kingRing[Us] & weak)
                  + 150 * popcount(pos.blockers_for_king(Us) | unsafeChecks)
-                 +       tropism * tropism / 4
+                 +   5 * tropism * tropism / 16
                  - 873 * !pos.count<QUEEN>(Them)
                  -   6 * mg_value(score) / 8
                  +       mg_value(mobility[Them] - mobility[Us])