Lock dpl version (v1.8.47)

It seems dpl v1.9 still has problems deploying to Bintray in our setup.

This issue is somewhat related to travis-ci/travis-ci#9314

Author: fniephaus

deploy/bintray.sh

@@ -18,5 +18,5 @@ if [[ "${APPVEYOR}" ]]; then
     export CMDSHELL="cmd /C "
 fi
 
-$CMDSHELL gem install dpl
+$CMDSHELL gem install dpl --version 1.8.47
 $CMDSHELL dpl --provider=bintray --user=osvm --key=$BINTRAYAPIKEY --file=bintray.json --skip_cleanup=true