bump to 2.5.2

drop OTP-17 from travis since it's too old for rebar3 to run tests

Merge pull request ferd#73 from ferd/various-fixes

Various fixes

Prepare release 2.4.0

Merge pull request ferd#66 from ferd/otp-21-support

OTP-21 support

Merge branch 'zhongwencool-master'

Bump to 2.3.4

Bump to 2.3.3

Cut 2.3.2

Bump to 2.3.1

Also fix hex packaging