Fix for OS X build.

Changed so that dependency is on the Accelerate framework, instead of
the deprecated vecLib framework. This caused `go install` to fail with the
following message on, at least, OS X Yosemite:

```
ld: framework not found vecLib
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```
Merge pull request #24 from pontusmelke/fix-osx-build

Fix for OS X build.