Build error related to eigen3

I tried to build dynet on macOS 10.13.5 using Xcode 9.4.1 and Apple LLVM version 9.1.0 (clang-902.0.39.2). I followed [the steps](https://github.com/clab/dynet), and did:

1. `homebrew install --HEAD eigen`
1. `cmake .. -DEIGEN3_INCLUDE_DIR=/usr/local/eigen3 -DENABLE_CPP_EXAMPLES=ON`
1. `make`

Then I got many replica of the following error:

```
/usr/local/Cellar/eigen/3.3.3/include/eigen3/unsupported/Eigen/CXX11/src/Tensor/TensorStorage.h:39:7: error: class template partial
specialization is not more specialized than the primary template [-Winvalid-partial-specialization]
class TensorStorage
^
/usr/local/Cellar/eigen/3.3.3/include/eigen3/unsupported/Eigen/CXX11/src/Tensor/TensorStorage.h:34:63: note: template is declared
here
template class TensorStorage;
```

Reproduce the reported build using the listed macOS, Xcode, Homebrew Eigen, CMake, and make commands. Start with the Eigen TensorStorage.h diagnostic and the dynet CMake configuration; done means the project builds successfully with the reported setup or the compatibility requirement is documented.

Written by the indexing model from the issue text.