Revert to GMP 6.1.1 as GMP 6.1.2 cannot be compiled on Travis-CI

scripts/bootstrap-depends.sh

@@ -9,7 +9,7 @@ LIBUV_TAG="v1.10.2"
 LIBUV_URL="https://github.com/libuv/libuv.git"
 KNOT_TAG="v2.3.3"
 KNOT_URL="https://github.com/CZ-NIC/knot.git"
-GMP_TAG="6.1.2"
+GMP_TAG="6.1.1" # GMP 6.1.2 is broken on Travis-CI
 GMP_URL="https://gmplib.org/download/gmp/gmp-${GMP_TAG}.tar.xz"
 JANSSON_TAG="2.9"
 JANSSON_URL="http://www.digip.org/jansson/releases/jansson-${JANSSON_TAG}.tar.gz"