Safe eval on 5.10 & 5.12

The call to eval on EventEmitter can clobber $@ on perl 5.10 and 5.12.

The common case, I believe, is client code using Mojo::UserAgent and die-ing under certain conditions but M::UA destructor triggers the eval on EventEmitter, clobbering the original exception.

Failing test results from Travis: https://travis-ci.org/carloslima/mojo/builds/13918616
Fixed test results: https://travis-ci.org/carloslima/mojo/builds/13918184