Fixed eigen.podspec and updated vMAT.podspec with eigen dependency

The **eigen** podspec was broken before:
```shell
aoide:vMAT kaelin$ pod spec lint /Users/Shared/github/Specs/eigen/3.1.2/eigen.podspec --no-clean

 -> eigen (3.1.2)
    - ERROR | The `preserve_paths` pattern did not match any file.

Pods project available at `/tmp/CocoaPods/Lint/Pods/Pods.xcodeproj` for inspection.

Analyzed 1 podspec.

[!] The spec did not pass validation.
```

Now it passes lint, and more importantly actually installs the **C++** template header files:
```shell
aoide:~ kaelin$ pod spec lint ~/.cocoapods/kaelin/eigen/3.1.2/eigen.podspec 

 -> eigen (3.1.2)

Analyzed 1 podspec.

eigen.podspec passed validation.
```