Remove scala-js 1.x support for now

Running into issues I can explain in a subsequent PR…

Author: alexarchambault

.travis.yml

@@ -13,7 +13,6 @@ scala:
   - 2.13.0-M1
 env:
   - SCALAJS_VERSION=0.6.23
-  - SCALAJS_VERSION=1.0.0-M1
 matrix:
   include:
     - scala: 2.10.6