build: bump to bitcoind 0.18.1 for itests

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 cache:
   directories:
-    - ~/bitcoin/bitcoin-0.18.0/bin
+    - ~/bitcoin/bitcoin-0.18.1/bin
     - $GOCACHE
     - $GOPATH/pkg/mod
     - $GOPATH/src/github.com/btcsuite

scripts/install_bitcoind.sh

@@ -2,7 +2,7 @@
 
 set -ev
 
-export BITCOIND_VERSION=0.18.0
+export BITCOIND_VERSION=0.18.1
 
 if sudo cp ~/bitcoin/bitcoin-$BITCOIND_VERSION/bin/bitcoind /usr/local/bin/bitcoind
 then