travis: try to reduce shelltestrunner rebuilding

.travis.yml

@@ -32,7 +32,9 @@ sudo: false
 cache:
   directories:
     - $HOME/.stack/
+    - $HOME/.local/bin/
     - $HOME/shelltestrunner/
+    - $HOME/shelltestrunner/.stack-work/
 
 # addons:
 #   apt:
@@ -70,10 +72,9 @@ script:
   # build hledger only, for testing config
   # - stack +RTS -N2 -RTS build --ghc-options=-Werror hledger
 
-  # build all packages, ensuring no warnings, no haddock failures, per-package test suites passing 
+  # build all packages, ensuring no warnings, no haddock failures, all package test suites passing 
   - stack +RTS -N2 -RTS build --ghc-options=-Werror --haddock --no-haddock-deps --test
-  # install hledger addon deps, build addons
-  # TODO: require successful warning-free builds
+  # install hledger addon deps, build addons (TODO: require successful warning-free builds)
   - bin/compile.sh
   # run functional tests
   - make functest