remove java 7 build

[xuwei-k]

• sbt 1.x does not work with Java 7
• travis-ci no longer support oraclejdk7 Trusty with oraclejdk7, ends with: ERROR: JAVA_HOME is set to an invalid directory: /usr/lib/jvm/java-7-oracle travis-ci/travis-ci#7884 (comment)

[codecov-io]

Codecov Report

Merging #2164 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2164   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files         328      328           
  Lines        5548     5548           
  Branches      213      213           
=======================================
  Hits         5252     5252           
  Misses        296      296

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[LukaJCB]

Thank you! :)

[kailuowang]

Thanks!