tools: travis: try a workaround after version bump failure

.travis.yml

@@ -107,6 +107,9 @@ install: # command output is hidden as they complete
   - if [[ ! -x ~/.local/bin/shelltest ]]; then stack install --stack-yaml=shelltestrunner/stack.yaml --resolver=lts-8.6; fi
   - shelltest --version
 
+  # try this to work around a strange failure after hledger version change
+  - stack clean
+
 script:  # command output is not hidden
 
   # less slow