Add cmake as a pip install dependency.

setup.py

@@ -231,6 +231,7 @@ setup(
     cmdclass=dict(build_ext=CMakeBuild, test=PyTest),
     zip_safe=False,
     tests_require=['pytest==3.8'],
+    install_requires=['cmake'],
     packages=['dlib'],
     keywords=['dlib', 'Computer Vision', 'Machine Learning'],
     classifiers=[