i686 mismatched types

[jaemk]

Getting an issue on travis, compiling for i686-apple-darwin

error[E0308]: mismatched types

   --> /Users/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/security-framework-0.1.15/src/trust.rs:124:69

    |

124 |             let certificate = SecTrustGetCertificateAtIndex(self.0, ix);

    |                                                                     ^^ expected i32, found i64

[sfackler]

@jchien14