Additions to docs/rtd-pip-requirements for readthedocs

[cdeil]

After merging #44 there's now this error on readthedocs:

/var/build/user_builds/photutils/envs/latest/src/astropy/astropy/sphinx/conf.py:156: UserWarning: matplotlib's plot_directive could not be imported. Inline plots will not be included in the output
  "Inline plots will not be included in the output")
WARNING: ConfigurationDefaultMissingWarning: Could not determine version of package photutils Cannot install default profile. If you are importing from source, this is expected. [photutils._astropy_init]
/var/build/user_builds/photutils/checkouts/latest/docs/photutils/findstars.rst:59: ERROR: Unknown directive type "plot".

See full log here: https://readthedocs.org/builds/photutils/1390241/
And the image here doesn't appear.

I think all that's missing is adding matplotlib to the docs/rtd-pip-requirements file, so I'll try that now and also add scipy and scikit-image because the example plotting code runs daofind and will need those.

cc @larrybradley

[coveralls]

Coverage remained the same when pulling 64ec48e on cdeil:issue_58 into 9769fab on astropy:master.