MNT: remove duplicate registration of colormaps

[tacaswell]

Both dimgray and dimgray_r are registering in the mpl_cm and do not need to be registered again.

Discovered this when looking at making register_cmap stricter and failing on duplicate registrations.

[tacaswell]

The test failure seems to be issues with travis configuration...

Encrypted environment variables have been removed for security reasons.
181See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
182
183Setting environment variables from .travis.yml
184$ export CHANS_DEV="-c pyviz/label/dev"
185$ export CHANS_REL="-c pyviz"
186$ export LABELS_DEV="--label dev"
187$ export LABELS_REL="--label dev --label main"
188$ export PYENV_VERSION=3.6
189$ export PKG_TEST_PYTHON="--test-python=py36 --test-python=py27"
190
191$ bash -c 'echo $BASH_VERSION'
1924.3.48(1)-release
193
1940.03s$ pip install pyctdev && doit miniconda_install && pip uninstall -y doit pyctdev
195pyenv: version `3.6' is not installed (set by PYENV_VERSION environment variable)
196The command "pip install pyctdev && doit miniconda_install && pip uninstall -y doit pyctdev" failed and exited with 1 during .
197
198Your build has been stopped.

[jsignell]

I can't push to your branch, so I took the commits and did a small fix. Thanks @tacaswell!

[tacaswell]

Weird, the "allow commits from maintainers" box is not checked, I do not remember un-checking it...