Install twine without `--user` for PyPI deployment
Install twine without `--user` for PyPI deployment

twine does not work well with virtualenv, and this caused issues
such as https://github.com/travis-ci/dpl/issues/296 and
https://github.com/travis-ci/dpl/issues/299.

This commit aims to fix the problem by installing twine without
`--user`, which is suggested in https://github.com/travis-ci/dpl/issues/299#issuecomment-130300309

flowlo/dpl,LoicMahieu/dpl,clayreimann/dpl,felixrieseberg/dpl,travis-ci/dpl,mathiasrw/dpl,axelfontaine/dpl,pengsrc/dpl,testfairy/dpl,Zyko0/dpl,mattk42/dpl,travis-ci/dpl,testfairy/dpl,computology/dpl,testfairy/dpl,kalbasit/dpl,Misfit-SW-China/dpl,georgantasp/dpl,jonathan-s/dpl,sebest/dpl,jsloyer/dpl,johanneswuerbach/dpl,travis-ci/dpl