appveyor.yml: use -j1, in hopes that this will help with a GHC bug.

The bug: https://ghc.haskell.org/trac/ghc/ticket/13194
See https://ci.appveyor.com/api/buildjobs/02l7v73n5hjs2t5u/log
for its occurrence in previous appveyor builds of pandoc.

appveyor.yml

@@ -30,8 +30,8 @@ test_script:
   # The ugly echo "" hack is to avoid complaints about 0 being an invalid file
   # descriptor
   - echo "" | stack clean
-  - echo "" | stack --no-terminal  test
-  - echo "" | stack --local-bin-path . install pandoc pandoc-citeproc
+  - echo "" | stack -j1 --no-terminal  test
+  - echo "" | stack -j1 --local-bin-path . install pandoc pandoc-citeproc
 
 after_test:
     # .\ in the stack commandline seems to be .\windows\ (where the stack-appveyor.yaml is)