Travis: decrease Windows itest parallelism

The Windows virtual machine that Travis runs the integration tests on
seems to be slower than the other machines. We try to increase the
stability of the tests by cutting the number of parallel running suites
in half. This will come at the cost of longer execution time but
hopefully with a better stability in return.

.travis.yml

@@ -77,7 +77,9 @@ jobs:
 
     - name: Btcd Integration Windows
       script:
-        - make itest-parallel windows=1
+        # The windows VM seems to be slower than the other Travis VMs. We only
+        # run 2 test suites in parallel instead of the default 4.
+        - make itest-parallel windows=1 ITEST_PARALLELISM=2
       os: windows
       before_install:
         - choco upgrade --no-progress -y make netcat curl findutils