external: Update jsoncpp revision

Bump JsonCpp external reference from version 1.7.7 to tag 1.8.0

Change-Id: I90ab630870f9417ab8bc3d719c0d13c05b12d5e2

external_revisions/jsoncpp_revision

diff --git a/external_revisions/jsoncpp_revision b/external_revisions/jsoncpp_revision
index f2f09db..27f9cd3 100644
--- a/external_revisions/jsoncpp_revision
+++ b/external_revisions/jsoncpp_revision
@@ -1 +1 @@
-d8cd848ede1071a25846cd90b4fddf269d868ff1
\ No newline at end of file
+1.8.0