bump Scala.js version in Travis-CI config, too

Author: SethTisue

.travis.yml

@@ -29,13 +29,13 @@ env:
   matrix:
     # The empty SCALAJS_VERSION will only compile for the JVM
     - SCALAJS_VERSION=
-    - SCALAJS_VERSION=0.6.25
+    - SCALAJS_VERSION=0.6.26
     - SCALAJS_VERSION=1.0.0-M6
 
 matrix:
   exclude:
     - jdk: openjdk11
-      env: SCALAJS_VERSION=0.6.25
+      env: SCALAJS_VERSION=0.6.26
     - jdk: openjdk11
       env: SCALAJS_VERSION=1.0.0-M6
     - scala: 2.11.12