Update notebooks that use reference file symbolic links to not be absolute and therefore not gh-page-able

[eteq]

I worked around this with a manual deploy for now, but the Initialization and wfpc2 drizzlepac notebooks both create iref files that point to absolute paths as symbolic links. This doesn't work for github pages. So the fix is to exclude the reference files (probably a good idea anyway) with a gitignore pattern, or fix the links so that they point to relative paths

[mackjenn]

@eteq We originally had these as relative paths: os.environ['iref'] = './reference_files/references/hst/wfc3/'

'Updated with suggestions from Erik regarding the path issues Travis is having. Placed absolute paths in specific places and updated the shell call to return information if it fails to run in the cell.' https://github.com/spacetelescope/notebooks/commit/130d7e5

For reference... https://github.com/spacetelescope/notebooks/pull/41#pullrequestreview-187262923

Let us know if you find a solution!