Merge #782: Check if variable=yes instead of if var is set in travis.sh

Merge #782: Check if variable=yes instead of if var is set in travis.sh

34debf7a6d36bbd9a52e68e079ddfc446faf5bef Modify .travis.yml to explictly pass no in env vars instead of setting to nothing (Elichai Turkel)
ef37761feed0172baa03dd94c842f1547bdf3016 Change travis.sh to check if variables are equal to yes instead of not-empty. Before this, setting `VALGRIND=wat` was considered as true, and to make it evaluate as false you had to unset the variable `VALGRIND=` but not it checks if `VALGRIND=yes` and if it's not `yes` then it's evaluated to false (Elichai Turkel)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 34debf7a6d36bbd9a52e68e079ddfc446faf5bef
  jonasnick:
    ACK 34debf7a6d36bbd9a52e68e079ddfc446faf5bef

Tree-SHA512: 91becfbc9cb7587ee55b2bceb604ea0aed8860990d63a5f414b11db92180c090ea8bcc048c2fb67a094e892138e3be46f00562bf78b7c3369232457289cde447