set dummy environment variable to work around Travis-CI cache issue

see #335 for details. at some point we should be able to remove this

Author: SethTisue

.travis.yml

@@ -11,9 +11,9 @@ scala:
   - 2.13.4
 
 env:
-  - ADOPTOPENJDK=8     SCALAJS_VERSION=
-  - ADOPTOPENJDK=8     SCALAJS_VERSION=1.3.1
-  - ADOPTOPENJDK=11    SCALAJS_VERSION=
+  - ADOPTOPENJDK=8     SCALAJS_VERSION=       CACHE_WORKAROUND=see-PR-335
+  - ADOPTOPENJDK=8     SCALAJS_VERSION=1.3.1  CACHE_WORKAROUND=see-PR-335
+  - ADOPTOPENJDK=11    SCALAJS_VERSION=       CACHE_WORKAROUND=see-PR-335
 
 matrix:
   include: