June 2018 release

New Levels:

1. Psychlab.

   1. contributed/psychlab/glass_pattern_detection
   2. contributed/psychlab/landoltC_identification
   3. contributed/psychlab/motion_discrimination{,_easy}
   4. contributed/psychlab/multiple_object_tracking{,_easy}
   5. contributed/psychlab/odd_one_out

Bug Fixes:

1. Let Python level cache set to None mean the same as not setting it at all.
2. Change Python module initialization in Python-3 mode to make PIP packages work in Python 3.

Minor Improvements:

1. Add support for absl::variant to lua::Push and lua::Read.
2. The demo :game has a new flag --start_index to start at an episode index other than 0.
3. Add a console command dm_pickup to pick up an item identified by its id.
4. More Python demos and tests now work with Python 3.
5. Add a shader for rendering decals with transparency.