Use OpenCV version actually working on arm64

I found your latest commit not working on arm64. Seems like a wrong `opencv2.framework` got included, since it doesn't include arm64. So I downloaded the framework from your link (http://sourceforge.net/projects/opencvlibrary/files/opencv-ios/2.4.7/opencv2.framework.zip/download) provided and replaced the other one. Now everything seems fine. I also updated the podspec file.

At a5c376988ffc6206efc836ddc33ed03044a7be43 (missing 64bit):
```
$ file opencv2.framework/Versions/A/opencv2 
opencv2.framework/Versions/A/opencv2: Mach-O universal binary with 3 architectures
opencv2.framework/Versions/A/opencv2 (for architecture armv7):	current ar archive random library
opencv2.framework/Versions/A/opencv2 (for architecture armv7s):	current ar archive random library
opencv2.framework/Versions/A/opencv2 (for architecture i386):	current ar archive random library
```

At 97b02c50ea0599619017f66da48157decb691dbd (with 64bit):
```
$ file opencv2.framework/Versions/A/opencv2 
opencv2.framework/Versions/A/opencv2: Mach-O universal binary with 5 architectures
opencv2.framework/Versions/A/opencv2 (for architecture cputype (16777228) cpusubtype (0)):	current ar archive random library
opencv2.framework/Versions/A/opencv2 (for architecture armv7):	current ar archive random library
opencv2.framework/Versions/A/opencv2 (for architecture armv7s):	current ar archive random library
opencv2.framework/Versions/A/opencv2 (for architecture i386):	current ar archive random library
opencv2.framework/Versions/A/opencv2 (for architecture x86_64):	current ar archive random library
```