remove outdated CI matrix exclusion

as noticed by Ol' Eagle Eye (whose initials are S.D.)

Author: SethTisue

.travis.yml

@@ -26,9 +26,6 @@ matrix:
     - jdk: oraclejdk8
       scala: 2.11.12
 
-    # 2.13.0-M5 is not available in Scala.js 1.0.0-M6
-    - scala: 2.13.0-M5
-      env: SCALAJS_VERSION=1.0.0-M6
   include:
 
     # run migration test