Update the link to the eigen (#456)

Downgrade the Sphinx to use 1.5.6

cmake/external/eigen.cmake

@@ -7,7 +7,7 @@ INCLUDE_DIRECTORIES(${EIGEN_SOURCE_DIR}/src/extern_eigen3)
 ExternalProject_Add(
     extern_eigen3
     ${EXTERNAL_PROJECT_LOG_ARGS}
-    GIT_REPOSITORY  "https://github.com/RLovelett/eigen.git"
+    GIT_REPOSITORY  "https://github.com/libigl/eigen.git"
     GIT_TAG         "master"
     PREFIX          ${EIGEN_SOURCE_DIR}
     UPDATE_COMMAND  ""

requirements.txt

 numpy==1.14.0
 recommonmark==0.4.0
 scipy==1.0.0
-Sphinx==1.6.6
+Sphinx==1.5.6
 sphinx-rtd-theme==0.2.4
 flake8==3.5.0
 Pillow==5.0.0