remove json.hpp from repo

.gitignore

@@ -13,4 +13,5 @@ Makefile.in
 .DS_Store
 *~
 /*/env
-/*/build
+/*/build
+/Grid/json.hpp

Grid/bootstrap.sh

@@ -2,5 +2,10 @@
 
 set -euo pipefail
 
+json_url='https://github.com/nlohmann/json/blob/bc889afb4c5bf1c0d8ee29ef35eaaf4c8bef8a5d/single_include/nlohmann/json.hpp'
+
+if [ ! -f json.hpp ]; then
+  wget ${json_url}
+fi
 mkdir -p .buildutils/m4
 autoreconf -fvi