Build for Scala 2.11 only at mater branch commits

[xerial]

Spark 2.4.x is rolled back to Scala 2.11 because there are many Spark users who still need time to migrate to Scala 2.12.

This PR will recover Scala 2.11 build only for master branch commits.

[codecov]

Codecov Report

Merging #498 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #498   +/-   ##
=======================================
  Coverage   83.66%   83.66%           
=======================================
  Files         209      209           
  Lines        7942     7942           
  Branches      608      608           
=======================================
  Hits         6645     6645           
  Misses       1297     1297

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a478d98...43a0e12. Read the comment docs.

[xerial]

cc: @takezoe