Revert "Let travis compile debug binaries so ASSERTS are also considered"

This reverts commit b1de41ca49d44eb5068d60c29f7960d363aadd75.

Led to unstable test results for no reason

See

https://travis-ci.org/qgis/QGIS/builds/42811273

Different results for the very same commit:
http://dash.orfeo-toolbox.org/buildSummary.php?buildid=165433
http://dash.orfeo-toolbox.org/buildSummary.php?buildid=165427