support using env var in travis ci for bintray upload
support using env var in travis ci for bintray upload

bugsnag/bugsnag-android-gradle-plugin,bugsnag/bugsnag-android-gradle-plugin,bugsnag/bugsnag-android-gradle-plugin,bugsnag/bugsnag-android-gradle-plugin,bugsnag/bugsnag-android-gradle-plugin,bugsnag/bugsnag-android-gradle-plugin