Specify encoding for reading notebook file.

Closes gh-4562
Specify encoding in remainining instances of io.open
Fixing cell_type in CodeCell constructor.

This was causing the cell_type to be null for CodeCells, which
broke the cell type dropdown in the toolbar.
Merge pull request #4563 from takluyver/i4562

Specify encoding for reading notebook file. and a few other calls to io.open with missing encoding
Merge pull request #4566 from ellisonbg/cell-type-fix

Fixing cell_type in CodeCell constructor.
no longer need 3 suffix on travis, tox
Merge pull request #4571 from minrk/travis-no-suffix

no longer need 3 suffix on travis, tox