64Bit Target updated to iOS 7

If you try to build the facebook sdk with scripts/build_framework.sh
and the default project settings, the build process aborts with the
message:

error: invalid deployment target '6.0.0' for architecture 'arm64'
(requires '7.0.0' or later)
1 error generated.

** BUILD FAILED **

The following build commands failed:
ProcessPCH
/var/folders/fm/jtl_yzt55wq_rtvdt0d1yzfc0000gn/C/com.apple.DeveloperTool
s/5.0-5A1413/Xcode/SharedPrecompiledHeaders/facebook_ios_sdk_Prefix-dfvw
jzbcrxnuwddxjkudqnqllrma/facebook_ios_sdk_Prefix.pch.pch
facebook_ios_sdk_Prefix.pch normal arm64 objective-c
com.apple.compilers.llvm.clang.1_0.compiler
(1 failure)

FATAL: XCode build failed for platform: iphoneos.

This commit changes the deployment targets of Debug64 and Release64
from iOS 6.0 to iOS 7.0 so the build process works probably.