Release version 0.9.0 (#349)

Fix #293, enable compiler warnings in debug mode.

Update version.txt

First post-competition version of NLE.

Bump version to 0.7.3.

Merge pull request #181 from facebookresearch/eric/release-0.7.2

Release 0.7.2

Remove seeds from internal observation, add warning.

Unfortunately, xwaitforspace is in internal currently and it is
somewhat useful when auto-skipping of messages isn't turned on.

Bump version.

Merge pull request #160 from facebookresearch/heiner/addhow

Heiner/addhow

Release candidate branch for version 0.6.0. (#137)

Fix typo.