Update bundled openresty/luajit2 to v2.1-20200102

Update the version of openresty/luajit2 in the 1.15.8.x branch to
v2.1-20200102.

This is to allow a release of openresty that includes a version of
luajit that supports the s390x architecture to be created. Support for
s390x was added to openresty/luajit2 by this PR -

https://github.com/openresty/luajit2/pull/50

which is included in openresty/luajit2 release v2.1-20200102, however
the bundled release of openresty/luajit2 in the latest release of
openresty, version 1.15.8.3, is v2.1-20190507, which has no support
for s390x.

Commit added by Matthew Arnold <mattarno@uk.ibm.com>

util/mirror-tarballs

@@ -688,7 +688,7 @@ mv openresty-opm-* opm-$ver || exit 1
 
 #################################
 
-ver=2.1-20190507
+ver=2.1-20200102
 $root/util/get-tarball "https://github.com/openresty/luajit2/archive/v$ver.tar.gz" -O "LuaJIT-$ver.tar.gz" || exit 1
 tar -xzf LuaJIT-$ver.tar.gz || exit 1
 mv luajit2-* LuaJIT-$ver || exit 1