fix test_envs

replace release notes with 0.15.1, update Taxi env registration logic…

… to Taxi-v3

0.15.0 + release notes

update travis yaml

fix readme

bump version to 0.13.0, update release notes

bump version, update release notes in README.rst

Fixes fetch/slide environment. (openai#1511)

bump version to 0.12.4

increment version