Add cmake back as a pip dependency.

This dependency was explicitly removed two years ago because pip was
installing a broken cmake on some systems.  I'm adding the dependency
back in the hope that the pip copy of cmake has been fixed on all
systems by this point.

setup.py

@@ -233,7 +233,7 @@ setup(
     zip_safe=False,
     # We need an older more-itertools version because v6 broke pytest (for everyone, not just dlib)
     tests_require=['pytest==3.8', 'more-itertools<6.0.0'],
-    #install_requires=['cmake'], # removed because the pip cmake package is busted, maybe someday it will be usable.
+    install_requires=['cmake'],
     packages=['dlib'],
     package_dir={'': 'tools/python'},
     keywords=['dlib', 'Computer Vision', 'Machine Learning'],