requirements: Update README and renamed files to "requirements-dev.txt"

This commit reverts 6663f97 (rename the requirements file to match
documentation).

Since scikit-build is only required to build the distributions (either
souce or wheel), it should be in the requirements-dev.txt. Indeed, there
is no need to install scikit-build when installing the wheels.

Fixes #11

Author: jcfr

README.md

@@ -14,7 +14,7 @@ Build the SimpleITK Python wheel with the following command:
 
 ```
 mkvirtualenv build-sitk
-pip install -r requirements.txt
+pip install -r requirements-dev.txt
 python setup.py bdist_wheel
 ```