Fix OS X Rust Builds
Fix OS X Rust Builds

This change should allow Rust projects to build on OS X - builds currently fail with the following error:
```
$ rustc --version
dyld: Library not loaded: x86_64-apple-darwin/stage1/lib/rustlib/x86_64-apple-darwin/lib/librustc_driver-4e7c5e5c.dylib
Referenced from: /Users/travis/rust/bin/rustc
Reason: image not found
/Users/travis/build.sh: line 41: 954 Trace/BPT trap: 5 rustc --version
$ cargo --version
Process didn't exit successfully: `rustc -v verbose` (status=5)
--- stderr
dyld: Library not loaded: x86_64-apple-darwin/stage1/lib/rustlib/x86_64-apple-darwin/lib/librustc_driver-4e7c5e5c.dylib
Referenced from: /Users/travis/rust/bin/rustc
Reason: image not found
```
kidaa/travis-build,dirk/travis-build,kevmoo/travis-build,aceofspades/travis-build,lrowe/travis-build,akoeplinger/travis-build,wereHamster/travis-build,will/travis-build,aceofspades/travis-build,kevmoo/travis-build,wereHamster/travis-build,jhass/travis-build,tmccombs/travis-build,final-ci/travis-build,vlamanna/travis-build,Joshua-Anderson/travis-build,Acidburn0zzz/travis-build,tianon/travis-build,jhass/travis-build,gavioto/travis-build,gavioto/travis-build,dirk/travis-build,luoqii/travis-build,tmccombs/travis-build,final-ci/travis-build,Tiger66639/travis-build,akoeplinger/travis-build,Distelli/travis-build,will/travis-build,aceofspades/travis-build,Tiger66639/travis-build,craigcitro/travis-build,vinaykaradia/travisBuild-clone,whip112/travis-build,will/travis-build,kidaa/travis-build,Distelli/travis-build,JuliaCI/travis-build,luoqii/travis-build,andyli/travis-build,paultcochrane/travis-build,vlamanna/travis-build,Joshua-Anderson/travis-build,Joshua-Anderson/travis-build,JuliaCI/travis-build,craigcitro/travis-build,paultcochrane/travis-build,paultcochrane/travis-build,whip112/travis-build,wereHamster/travis-build,vinaykaradia/travisBuild-clone,Acidburn0zzz/travis-build,final-ci/travis-build,tianon/travis-build,andyli/travis-build,vlamanna/travis-build,lrowe/travis-build,vinaykaradia/travisBuild-clone